diff --git a/RefactorThis.Domain.Tests/InvoicePaymentProcessorTests.cs b/RefactorThis.Domain.Tests/InvoicePaymentProcessorTests.cs
index 3a607fd..97f10be 100644
--- a/RefactorThis.Domain.Tests/InvoicePaymentProcessorTests.cs
+++ b/RefactorThis.Domain.Tests/InvoicePaymentProcessorTests.cs
@@ -1,247 +1,241 @@
-using System;
+using NUnit.Framework;
+using RefactorThis.Domain.Service;
+using RefactorThis.Persistence.IRepository;
+using RefactorThis.Persistence.Models;
+using RefactorThis.Persistence.Repository;
+using System;
 using System.Collections.Generic;
-using NUnit.Framework;
-using RefactorThis.Persistence;
 
 namespace RefactorThis.Domain.Tests
 {
 	[TestFixture]
-	public class InvoicePaymentProcessorTests
-	{
-		[Test]
-		public void ProcessPayment_Should_ThrowException_When_NoInoiceFoundForPaymentReference( )
-		{
-			var repo = new InvoiceRepository( );
-
-			Invoice invoice = null;
-			var paymentProcessor = new InvoiceService( repo );
-
-			var payment = new Payment( );
-			var failureMessage = "";
-
-			try
-			{
-				var result = paymentProcessor.ProcessPayment( payment );
-			}
-			catch ( InvalidOperationException e )
-			{
-				failureMessage = e.Message;
-			}
-
-			Assert.AreEqual( "There is no invoice matching this payment", failureMessage );
-		}
-
-		[Test]
-		public void ProcessPayment_Should_ReturnFailureMessage_When_NoPaymentNeeded( )
-		{
-			var repo = new InvoiceRepository( );
-
-			var invoice = new Invoice( repo )
-			{
-				Amount = 0,
-				AmountPaid = 0,
-				Payments = null
-			};
-
-			repo.Add( invoice );
-
-			var paymentProcessor = new InvoiceService( repo );
-
-			var payment = new Payment( );
-
-			var result = paymentProcessor.ProcessPayment( payment );
-
-			Assert.AreEqual( "no payment needed", result );
-		}
-
-		[Test]
-		public void ProcessPayment_Should_ReturnFailureMessage_When_InvoiceAlreadyFullyPaid( )
-		{
-			var repo = new InvoiceRepository( );
-
-			var invoice = new Invoice( repo )
-			{
-				Amount = 10,
-				AmountPaid = 10,
-				Payments = new List<Payment>
-				{
-					new Payment
-					{
-						Amount = 10
-					}
-				}
-			};
-			repo.Add( invoice );
-
-			var paymentProcessor = new InvoiceService( repo );
-
-			var payment = new Payment( );
-
-			var result = paymentProcessor.ProcessPayment( payment );
-
-			Assert.AreEqual( "invoice was already fully paid", result );
-		}
-
-		[Test]
-		public void ProcessPayment_Should_ReturnFailureMessage_When_PartialPaymentExistsAndAmountPaidExceedsAmountDue( )
-		{
-			var repo = new InvoiceRepository( );
-			var invoice = new Invoice( repo )
-			{
-				Amount = 10,
-				AmountPaid = 5,
-				Payments = new List<Payment>
-				{
-					new Payment
-					{
-						Amount = 5
-					}
-				}
-			};
-			repo.Add( invoice );
-
-			var paymentProcessor = new InvoiceService( repo );
-
-			var payment = new Payment( )
-			{
-				Amount = 6
-			};
-
-			var result = paymentProcessor.ProcessPayment( payment );
-
-			Assert.AreEqual( "the payment is greater than the partial amount remaining", result );
-		}
-
-		[Test]
-		public void ProcessPayment_Should_ReturnFailureMessage_When_NoPartialPaymentExistsAndAmountPaidExceedsInvoiceAmount( )
-		{
-			var repo = new InvoiceRepository( );
-			var invoice = new Invoice( repo )
-			{
-				Amount = 5,
-				AmountPaid = 0,
-				Payments = new List<Payment>( )
-			};
-			repo.Add( invoice );
-
-			var paymentProcessor = new InvoiceService( repo );
-
-			var payment = new Payment( )
-			{
-				Amount = 6
-			};
-
-			var result = paymentProcessor.ProcessPayment( payment );
-
-			Assert.AreEqual( "the payment is greater than the invoice amount", result );
-		}
-
-		[Test]
-		public void ProcessPayment_Should_ReturnFullyPaidMessage_When_PartialPaymentExistsAndAmountPaidEqualsAmountDue( )
-		{
-			var repo = new InvoiceRepository( );
-			var invoice = new Invoice( repo )
-			{
-				Amount = 10,
-				AmountPaid = 5,
-				Payments = new List<Payment>
-				{
-					new Payment
-					{
-						Amount = 5
-					}
-				}
-			};
-			repo.Add( invoice );
-
-			var paymentProcessor = new InvoiceService( repo );
-
-			var payment = new Payment( )
-			{
-				Amount = 5
-			};
-
-			var result = paymentProcessor.ProcessPayment( payment );
-
-			Assert.AreEqual( "final partial payment received, invoice is now fully paid", result );
-		}
-
-		[Test]
-		public void ProcessPayment_Should_ReturnFullyPaidMessage_When_NoPartialPaymentExistsAndAmountPaidEqualsInvoiceAmount( )
-		{
-			var repo = new InvoiceRepository( );
-			var invoice = new Invoice( repo )
-			{
-				Amount = 10,
-				AmountPaid = 0,
-				Payments = new List<Payment>( ) { new Payment( ) { Amount = 10 } }
-			};
-			repo.Add( invoice );
-
-			var paymentProcessor = new InvoiceService( repo );
-
-			var payment = new Payment( )
-			{
-				Amount = 10
-			};
-
-			var result = paymentProcessor.ProcessPayment( payment );
-
-			Assert.AreEqual( "invoice was already fully paid", result );
-		}
-
-		[Test]
-		public void ProcessPayment_Should_ReturnPartiallyPaidMessage_When_PartialPaymentExistsAndAmountPaidIsLessThanAmountDue( )
-		{
-			var repo = new InvoiceRepository( );
-			var invoice = new Invoice( repo )
-			{
-				Amount = 10,
-				AmountPaid = 5,
-				Payments = new List<Payment>
-				{
-					new Payment
-					{
-						Amount = 5
-					}
-				}
-			};
-			repo.Add( invoice );
-
-			var paymentProcessor = new InvoiceService( repo );
-
-			var payment = new Payment( )
-			{
-				Amount = 1
-			};
-
-			var result = paymentProcessor.ProcessPayment( payment );
-
-			Assert.AreEqual( "another partial payment received, still not fully paid", result );
-		}
-
-		[Test]
-		public void ProcessPayment_Should_ReturnPartiallyPaidMessage_When_NoPartialPaymentExistsAndAmountPaidIsLessThanInvoiceAmount( )
-		{
-			var repo = new InvoiceRepository( );
-			var invoice = new Invoice( repo )
-			{
-				Amount = 10,
-				AmountPaid = 0,
-				Payments = new List<Payment>( )
-			};
-			repo.Add( invoice );
-
-			var paymentProcessor = new InvoiceService( repo );
-
-			var payment = new Payment( )
-			{
-				Amount = 1
-			};
-
-			var result = paymentProcessor.ProcessPayment( payment );
-
-			Assert.AreEqual( "invoice is now partially paid", result );
-		}
-	}
+    public class InvoicePaymentProcessorTests
+    {
+        [Test]
+        public void ProcessPayment_Should_ThrowException_When_NoInoiceFoundForPaymentReference()
+        {
+            IInvoiceRepository repo = new InvoiceRepository();
+
+            var paymentProcessor = new InvoiceService(repo);
+
+            var payment = new Payment();
+
+            var result = paymentProcessor.ProcessPayment(payment);
+
+            Assert.AreEqual("There is no invoice matching this payment", result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFailureMessage_When_NoPaymentNeeded()
+        {
+            IInvoiceRepository repo = new InvoiceRepository();
+
+            var invoice = new Invoice()
+            {
+                Amount = 0,
+                AmountPaid = 0,
+                Payments = null
+            };
+
+            repo.Add(invoice);
+
+            var paymentProcessor = new InvoiceService(repo);
+
+            var payment = new Payment();
+
+            var result = paymentProcessor.ProcessPayment(payment);
+
+            Assert.AreEqual("no payment needed", result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFailureMessage_When_InvoiceAlreadyFullyPaid()
+        {
+            IInvoiceRepository repo = new InvoiceRepository();
+
+            var invoice = new Invoice()
+            {
+                Amount = 10,
+                AmountPaid = 10,
+                Payments = new List<Payment>
+                {
+                    new Payment
+                    {
+                        Amount = 10
+                    }
+                }
+            };
+            repo.Add(invoice);
+
+            var paymentProcessor = new InvoiceService(repo);
+
+            var payment = new Payment();
+
+            var result = paymentProcessor.ProcessPayment(payment);
+
+            Assert.AreEqual("invoice was already fully paid", result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFailureMessage_When_PartialPaymentExistsAndAmountPaidExceedsAmountDue()
+        {
+            IInvoiceRepository repo = new InvoiceRepository();
+            var invoice = new Invoice()
+            {
+                Amount = 10,
+                AmountPaid = 5,
+                Payments = new List<Payment>
+                {
+                    new Payment
+                    {
+                        Amount = 5
+                    }
+                }
+            };
+            repo.Add(invoice);
+
+            var paymentProcessor = new InvoiceService(repo);
+
+            var payment = new Payment()
+            {
+                Amount = 6
+            };
+
+            var result = paymentProcessor.ProcessPayment(payment);
+
+            Assert.AreEqual("the payment is greater than the partial amount remaining", result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFailureMessage_When_NoPartialPaymentExistsAndAmountPaidExceedsInvoiceAmount()
+        {
+            IInvoiceRepository repo = new InvoiceRepository();
+            var invoice = new Invoice()
+            {
+                Amount = 5,
+                AmountPaid = 0,
+                Payments = new List<Payment>()
+            };
+            repo.Add(invoice);
+
+            var paymentProcessor = new InvoiceService(repo);
+
+            var payment = new Payment()
+            {
+                Amount = 6
+            };
+
+            var result = paymentProcessor.ProcessPayment(payment);
+
+            Assert.AreEqual("the payment is greater than the invoice amount", result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFullyPaidMessage_When_PartialPaymentExistsAndAmountPaidEqualsAmountDue()
+        {
+            IInvoiceRepository repo = new InvoiceRepository();
+            var invoice = new Invoice()
+            {
+                Amount = 10,
+                AmountPaid = 5,
+                Payments = new List<Payment>
+                {
+                    new Payment
+                    {
+                        Amount = 5
+                    }
+                }
+            };
+            repo.Add(invoice);
+
+            var paymentProcessor = new InvoiceService(repo);
+
+            var payment = new Payment()
+            {
+                Amount = 5
+            };
+
+            var result = paymentProcessor.ProcessPayment(payment);
+
+            Assert.AreEqual("final partial payment received, invoice is now fully paid", result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnFullyPaidMessage_When_NoPartialPaymentExistsAndAmountPaidEqualsInvoiceAmount()
+        {
+            IInvoiceRepository repo = new InvoiceRepository();
+            var invoice = new Invoice()
+            {
+                Amount = 10,
+                AmountPaid = 0,
+                Payments = new List<Payment>() { new Payment() { Amount = 10 } }
+            };
+            repo.Add(invoice);
+
+            var paymentProcessor = new InvoiceService(repo);
+
+            var payment = new Payment()
+            {
+                Amount = 10
+            };
+
+            var result = paymentProcessor.ProcessPayment(payment);
+
+            Assert.AreEqual("invoice was already fully paid", result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnPartiallyPaidMessage_When_PartialPaymentExistsAndAmountPaidIsLessThanAmountDue()
+        {
+            IInvoiceRepository repo = new InvoiceRepository();
+            var invoice = new Invoice()
+            {
+                Amount = 10,
+                AmountPaid = 5,
+                Payments = new List<Payment>
+                {
+                    new Payment
+                    {
+                        Amount = 5
+                    }
+                }
+            };
+            repo.Add(invoice);
+
+            var paymentProcessor = new InvoiceService(repo);
+
+            var payment = new Payment()
+            {
+                Amount = 1
+            };
+
+            var result = paymentProcessor.ProcessPayment(payment);
+
+            Assert.AreEqual("another partial payment received, still not fully paid", result);
+        }
+
+        [Test]
+        public void ProcessPayment_Should_ReturnPartiallyPaidMessage_When_NoPartialPaymentExistsAndAmountPaidIsLessThanInvoiceAmount()
+        {
+            IInvoiceRepository repo = new InvoiceRepository();
+            var invoice = new Invoice()
+            {
+                Amount = 10,
+                AmountPaid = 0,
+                Payments = new List<Payment>()
+            };
+            repo.Add(invoice);
+
+            var paymentProcessor = new InvoiceService(repo);
+
+            var payment = new Payment()
+            {
+                Amount = 1
+            };
+
+            var result = paymentProcessor.ProcessPayment(payment);
+
+            Assert.AreEqual("invoice is now partially paid", result);
+        }
+    }
 }
\ No newline at end of file
diff --git a/RefactorThis.Domain.Tests/RefactorThis.Domain.Tests.csproj b/RefactorThis.Domain.Tests/RefactorThis.Domain.Tests.csproj
index f118007..c7e786d 100644
--- a/RefactorThis.Domain.Tests/RefactorThis.Domain.Tests.csproj
+++ b/RefactorThis.Domain.Tests/RefactorThis.Domain.Tests.csproj
@@ -1,67 +1,148 @@
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+    </Reference>
+    <Reference Include="System.Reflection.Metadata, Version=1.4.3.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
+      <HintPath>..\packages\System.Reflection.Metadata.1.6.0\lib\netstandard2.0\System.Reflection.Metadata.dll</HintPath>
+    </Reference>
+    <Reference Include="System.Runtime" />
+    <Reference Include="System.Runtime.CompilerServices.Unsafe, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
+      <HintPath>..\packages\System.Runtime.CompilerServices.Unsafe.5.0.0\lib\net45\System.Runtime.CompilerServices.Unsafe.dll</HintPath>
+    </Reference>
+    <Reference Include="System.Runtime.Serialization" />
+    <Reference Include="System.Xml" />
+    <Reference Include="nunit.framework, Version=3.5.0.0, Culture=neutral, PublicKeyToken=2638cd05610744eb">
+      <HintPath>..\packages\NUnit.3.5.0\lib\net45\nunit.framework.dll</HintPath>
+    </Reference>
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="InvoicePaymentProcessorTests.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\RefactorThis.Domain\RefactorThis.Domain.csproj">
+      <Project>{5310b2fe-e26d-414e-b656-1f74c5a70368}</Project>
+      <Name>RefactorThis.Domain</Name>
+    </ProjectReference>
+    <ProjectReference Include="..\RefactorThis.Persistence\RefactorThis.Persistence.csproj">
+      <Project>{33cdc796-ff75-449c-9637-59c2efc46361}</Project>
+      <Name>RefactorThis.Persistence</Name>
+    </ProjectReference>
+  </ItemGroup>
+  <ItemGroup>
+    <None Include="app.config" />
+    <None Include="packages.config" />
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+  <Target Name="EnsureNuGetPackageBuildImports" BeforeTargets="PrepareForBuild">
+    <PropertyGroup>
+      <ErrorText>This project references NuGet package(s) that are missing on this computer. Use NuGet Package Restore to download them.  For more information, see http://go.microsoft.com/fwlink/?LinkID=322105. The missing file is {0}.</ErrorText>
+    </PropertyGroup>
+    <Error Condition="!Exists('..\packages\Microsoft.Testing.Platform.1.7.3\build\netstandard2.0\Microsoft.Testing.Platform.props')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\Microsoft.Testing.Platform.1.7.3\build\netstandard2.0\Microsoft.Testing.Platform.props'))" />
+    <Error Condition="!Exists('..\packages\Microsoft.Testing.Platform.1.7.3\build\netstandard2.0\Microsoft.Testing.Platform.targets')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\Microsoft.Testing.Platform.1.7.3\build\netstandard2.0\Microsoft.Testing.Platform.targets'))" />
+    <Error Condition="!Exists('..\packages\Microsoft.Testing.Platform.MSBuild.1.7.3\build\Microsoft.Testing.Platform.MSBuild.props')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\Microsoft.Testing.Platform.MSBuild.1.7.3\build\Microsoft.Testing.Platform.MSBuild.props'))" />
+    <Error Condition="!Exists('..\packages\Microsoft.Testing.Platform.MSBuild.1.7.3\build\Microsoft.Testing.Platform.MSBuild.targets')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\Microsoft.Testing.Platform.MSBuild.1.7.3\build\Microsoft.Testing.Platform.MSBuild.targets'))" />
+    <Error Condition="!Exists('..\packages\Microsoft.Testing.Extensions.Telemetry.1.7.3\build\netstandard2.0\Microsoft.Testing.Extensions.Telemetry.props')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\Microsoft.Testing.Extensions.Telemetry.1.7.3\build\netstandard2.0\Microsoft.Testing.Extensions.Telemetry.props'))" />
+    <Error Condition="!Exists('..\packages\NUnit3TestAdapter.5.1.0\build\net462\NUnit3TestAdapter.props')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\NUnit3TestAdapter.5.1.0\build\net462\NUnit3TestAdapter.props'))" />
+    <Error Condition="!Exists('..\packages\NUnit3TestAdapter.5.1.0\build\net462\NUnit3TestAdapter.targets')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\NUnit3TestAdapter.5.1.0\build\net462\NUnit3TestAdapter.targets'))" />
+  </Target>
+  <Import Project="..\packages\Microsoft.Testing.Platform.1.7.3\build\netstandard2.0\Microsoft.Testing.Platform.targets" Condition="Exists('..\packages\Microsoft.Testing.Platform.1.7.3\build\netstandard2.0\Microsoft.Testing.Platform.targets')" />
+  <Import Project="..\packages\Microsoft.Testing.Platform.MSBuild.1.7.3\build\Microsoft.Testing.Platform.MSBuild.targets" Condition="Exists('..\packages\Microsoft.Testing.Platform.MSBuild.1.7.3\build\Microsoft.Testing.Platform.MSBuild.targets')" />
+  <Import Project="..\packages\NUnit3TestAdapter.5.1.0\build\net462\NUnit3TestAdapter.targets" Condition="Exists('..\packages\NUnit3TestAdapter.5.1.0\build\net462\NUnit3TestAdapter.targets')" />
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
 		 Other similar extension points exist, see Microsoft.Common.targets.
 	<Target Name="BeforeBuild">
 	</Target>
 	<Target Name="AfterBuild">
 	</Target>
 	-->
-
-</Project>
+</Project>
\ No newline at end of file
diff --git a/RefactorThis.Domain.Tests/app.config b/RefactorThis.Domain.Tests/app.config
new file mode 100644
index 0000000..f10b4ac
--- /dev/null
+++ b/RefactorThis.Domain.Tests/app.config
@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <runtime>
+    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
+      <dependentAssembly>
+        <assemblyIdentity name="System.Runtime.CompilerServices.Unsafe" publicKeyToken="b03f5f7f11d50a3a" culture="neutral" />
+        <bindingRedirect oldVersion="0.0.0.0-5.0.0.0" newVersion="5.0.0.0" />
+      </dependentAssembly>
+    </assemblyBinding>
+  </runtime>
+</configuration>
\ No newline at end of file
diff --git a/RefactorThis.Domain.Tests/packages.config b/RefactorThis.Domain.Tests/packages.config
index c108d44..76303f3 100644
--- a/RefactorThis.Domain.Tests/packages.config
+++ b/RefactorThis.Domain.Tests/packages.config
@@ -1,4 +1,20 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
+  <package id="Microsoft.ApplicationInsights" version="2.23.0" targetFramework="net472" />
+  <package id="Microsoft.Testing.Extensions.Telemetry" version="1.7.3" targetFramework="net472" />
+  <package id="Microsoft.Testing.Extensions.TrxReport.Abstractions" version="1.7.3" targetFramework="net472" />
+  <package id="Microsoft.Testing.Extensions.VSTestBridge" version="1.7.3" targetFramework="net472" />
+  <package id="Microsoft.Testing.Platform" version="1.7.3" targetFramework="net472" />
+  <package id="Microsoft.Testing.Platform.MSBuild" version="1.7.3" targetFramework="net472" />
+  <package id="Microsoft.TestPlatform.AdapterUtilities" version="17.13.0" targetFramework="net472" />
+  <package id="Microsoft.TestPlatform.ObjectModel" version="17.13.0" targetFramework="net472" />
   <package id="NUnit" version="3.5.0" targetFramework="net45" />
+  <package id="NUnit3TestAdapter" version="5.1.0" targetFramework="net472" />
+  <package id="System.Buffers" version="4.5.1" targetFramework="net472" />
+  <package id="System.Collections.Immutable" version="1.5.0" targetFramework="net472" />
+  <package id="System.Diagnostics.DiagnosticSource" version="5.0.0" targetFramework="net472" />
+  <package id="System.Memory" version="4.5.4" targetFramework="net472" />
+  <package id="System.Numerics.Vectors" version="4.5.0" targetFramework="net472" />
+  <package id="System.Reflection.Metadata" version="1.6.0" targetFramework="net472" />
+  <package id="System.Runtime.CompilerServices.Unsafe" version="5.0.0" targetFramework="net472" />
 </packages>
\ No newline at end of file
diff --git a/RefactorThis.Domain/InvoiceService.cs b/RefactorThis.Domain/InvoiceService.cs
deleted file mode 100644
index fbd674c..0000000
--- a/RefactorThis.Domain/InvoiceService.cs
+++ /dev/null
@@ -1,149 +0,0 @@
-using System;
-using System.Linq;
-using RefactorThis.Persistence;
-
-namespace RefactorThis.Domain
-{
-	public class InvoiceService
-	{
-		private readonly InvoiceRepository _invoiceRepository;
-
-		public InvoiceService( InvoiceRepository invoiceRepository )
-		{
-			_invoiceRepository = invoiceRepository;
-		}
-
-		public string ProcessPayment( Payment payment )
-		{
-			var inv = _invoiceRepository.GetInvoice( payment.Reference );
-
-			var responseMessage = string.Empty;
-
-			if ( inv == null )
-			{
-				throw new InvalidOperationException( "There is no invoice matching this payment" );
-			}
-			else
-			{
-				if ( inv.Amount == 0 )
-				{
-					if ( inv.Payments == null || !inv.Payments.Any( ) )
-					{
-						responseMessage = "no payment needed";
-					}
-					else
-					{
-						throw new InvalidOperationException( "The invoice is in an invalid state, it has an amount of 0 and it has payments." );
-					}
-				}
-				else
-				{
-					if ( inv.Payments != null && inv.Payments.Any( ) )
-					{
-						if ( inv.Payments.Sum( x => x.Amount ) != 0 && inv.Amount == inv.Payments.Sum( x => x.Amount ) )
-						{
-							responseMessage = "invoice was already fully paid";
-						}
-						else if ( inv.Payments.Sum( x => x.Amount ) != 0 && payment.Amount > ( inv.Amount - inv.AmountPaid ) )
-						{
-							responseMessage = "the payment is greater than the partial amount remaining";
-						}
-						else
-						{
-							if ( ( inv.Amount - inv.AmountPaid ) == payment.Amount )
-							{
-								switch ( inv.Type )
-								{
-									case InvoiceType.Standard:
-										inv.AmountPaid += payment.Amount;
-										inv.Payments.Add( payment );
-										responseMessage = "final partial payment received, invoice is now fully paid";
-										break;
-									case InvoiceType.Commercial:
-										inv.AmountPaid += payment.Amount;
-										inv.TaxAmount += payment.Amount * 0.14m;
-										inv.Payments.Add( payment );
-										responseMessage = "final partial payment received, invoice is now fully paid";
-										break;
-									default:
-										throw new ArgumentOutOfRangeException( );
-								}
-								
-							}
-							else
-							{
-								switch ( inv.Type )
-								{
-									case InvoiceType.Standard:
-										inv.AmountPaid += payment.Amount;
-										inv.Payments.Add( payment );
-										responseMessage = "another partial payment received, still not fully paid";
-										break;
-									case InvoiceType.Commercial:
-										inv.AmountPaid += payment.Amount;
-										inv.TaxAmount += payment.Amount * 0.14m;
-										inv.Payments.Add( payment );
-										responseMessage = "another partial payment received, still not fully paid";
-										break;
-									default:
-										throw new ArgumentOutOfRangeException( );
-								}
-							}
-						}
-					}
-					else
-					{
-						if ( payment.Amount > inv.Amount )
-						{
-							responseMessage = "the payment is greater than the invoice amount";
-						}
-						else if ( inv.Amount == payment.Amount )
-						{
-							switch ( inv.Type )
-							{
-								case InvoiceType.Standard:
-									inv.AmountPaid = payment.Amount;
-									inv.TaxAmount = payment.Amount * 0.14m;
-									inv.Payments.Add( payment );
-									responseMessage = "invoice is now fully paid";
-									break;
-								case InvoiceType.Commercial:
-									inv.AmountPaid = payment.Amount;
-									inv.TaxAmount = payment.Amount * 0.14m;
-									inv.Payments.Add( payment );
-									responseMessage = "invoice is now fully paid";
-									break;
-								default:
-									throw new ArgumentOutOfRangeException( );
-							}
-						}
-						else
-						{
-							switch ( inv.Type )
-							{
-								case InvoiceType.Standard:
-									inv.AmountPaid = payment.Amount;
-									inv.TaxAmount = payment.Amount * 0.14m;
-									inv.Payments.Add( payment );
-									responseMessage = "invoice is now partially paid";
-									break;
-								case InvoiceType.Commercial:
-									inv.AmountPaid = payment.Amount;
-									inv.TaxAmount = payment.Amount * 0.14m;
-									inv.Payments.Add( payment );
-									responseMessage = "invoice is now partially paid";
-									break;
-								default:
-									throw new ArgumentOutOfRangeException( );
-							}
-						}
-					}
-				}
-			}
-			
-			inv.Save();
-
-			return responseMessage;
-		}
-	}
-}
\ No newline at end of file
diff --git a/RefactorThis.Domain/RefactorThis.Domain.csproj b/RefactorThis.Domain/RefactorThis.Domain.csproj
index 753e893..320e623 100644
--- a/RefactorThis.Domain/RefactorThis.Domain.csproj
+++ b/RefactorThis.Domain/RefactorThis.Domain.csproj
@@ -1,59 +1,58 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-	<Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
-	<PropertyGroup>
-		<Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
-		<Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
-		<ProjectGuid>{5310B2FE-E26D-414E-B656-1F74C5A70368}</ProjectGuid>
-		<OutputType>Library</OutputType>
-		<AppDesignerFolder>Properties</AppDesignerFolder>
-		<RootNamespace>RefactorThis.Domain</RootNamespace>
-		<AssemblyName>RefactorThis.Domain</AssemblyName>
-		<TargetFrameworkVersion>v4.7.2</TargetFrameworkVersion>
-		<FileAlignment>512</FileAlignment>
-	</PropertyGroup>
-	<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
-		<PlatformTarget>AnyCPU</PlatformTarget>
-		<DebugSymbols>true</DebugSymbols>
-		<DebugType>full</DebugType>
-		<Optimize>false</Optimize>
-		<OutputPath>bin\Debug\</OutputPath>
-		<DefineConstants>DEBUG;TRACE</DefineConstants>
-		<ErrorReport>prompt</ErrorReport>
-		<WarningLevel>4</WarningLevel>
-	</PropertyGroup>
-	<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
-		<PlatformTarget>AnyCPU</PlatformTarget>
-		<DebugType>pdbonly</DebugType>
-		<Optimize>true</Optimize>
-		<OutputPath>bin\Release\</OutputPath>
-		<DefineConstants>TRACE</DefineConstants>
-		<ErrorReport>prompt</ErrorReport>
-		<WarningLevel>4</WarningLevel>
-	</PropertyGroup>
-	<ItemGroup>
-		<Reference Include="System" />
-		<Reference Include="System.Core" />
-		<Reference Include="System.Data" />
-		<Reference Include="System.Xml" />
-	</ItemGroup>
-	<ItemGroup>
-		<Compile Include="InvoiceService.cs" />
-		<Compile Include="Properties\AssemblyInfo.cs" />
-	</ItemGroup>
-	<ItemGroup>
-	  <ProjectReference Include="..\RefactorThis.Persistence\RefactorThis.Persistence.csproj">
-	    <Project>{33cdc796-ff75-449c-9637-59c2efc46361}</Project>
-	    <Name>RefactorThis.Persistence</Name>
-	  </ProjectReference>
-	</ItemGroup>
-	<Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
-	<!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProjectGuid>{5310B2FE-E26D-414E-B656-1F74C5A70368}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>RefactorThis.Domain</RootNamespace>
+    <AssemblyName>RefactorThis.Domain</AssemblyName>
+    <TargetFrameworkVersion>v4.7.2</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <PlatformTarget>AnyCPU</PlatformTarget>
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <PlatformTarget>AnyCPU</PlatformTarget>
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="System" />
+    <Reference Include="System.Core" />
+    <Reference Include="System.Data" />
+    <Reference Include="System.Xml" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="Service\InvoiceService.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\RefactorThis.Persistence\RefactorThis.Persistence.csproj">
+      <Project>{33cdc796-ff75-449c-9637-59c2efc46361}</Project>
+      <Name>RefactorThis.Persistence</Name>
+    </ProjectReference>
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
 		 Other similar extension points exist, see Microsoft.Common.targets.
 	<Target Name="BeforeBuild">
 	</Target>
 	<Target Name="AfterBuild">
 	</Target>
 	-->
-
-</Project>
+</Project>
\ No newline at end of file
diff --git a/RefactorThis.Domain/Service/InvoiceService.cs b/RefactorThis.Domain/Service/InvoiceService.cs
new file mode 100644
index 0000000..f41b492
--- /dev/null
+++ b/RefactorThis.Domain/Service/InvoiceService.cs
@@ -0,0 +1,150 @@
+using RefactorThis.Persistence;
+using RefactorThis.Persistence.IRepository;
+using RefactorThis.Persistence.Models;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace RefactorThis.Domain.Service
+{
+    public class InvoiceService
+    {
+        private readonly IInvoiceRepository _invoiceRepository;
+
+        public InvoiceService(IInvoiceRepository invoiceRepository)
+        {
+            _invoiceRepository = invoiceRepository;
+        }
+
+        public string ProcessPayment(Payment payment)
+        {
+            var inv = _invoiceRepository.GetInvoice(payment.Reference);
+
+            var responseMessage = string.Empty;
+
+            if (inv == null)
+            {
+                responseMessage = "There is no invoice matching this payment";
+            }
+            else
+            {
+                ValidateAndProcessInvoice(inv, payment, ref responseMessage);
+            }
+
+            _invoiceRepository.SaveInvoice(inv);
+
+            return responseMessage;
+        }
+
+        private void ValidateAndProcessInvoice(Invoice inv, Payment payment, ref string responseMessage)
+        {
+            if (inv.Amount == 0)
+            {
+                responseMessage = (inv.Payments == null || !inv.Payments.Any()) ? "no payment needed" : "The invoice is in an invalid state, it has an amount of 0 and it has payments.";
+            }
+            else
+            {
+                ProcessInvoicePayment(inv, payment, ref responseMessage);
+            }
+        }
+
+        private void ProcessInvoicePayment(Invoice inv, Payment payment, ref string responseMessage)
+        {
+            if (inv.Payments != null && inv.Payments.Any())
+            {
+                HandleInvoiceWithBalancePayments(inv, payment, ref responseMessage);
+
+            }
+            else
+            {
+                HandleNonPaidInvoice(inv, payment, ref responseMessage);
+            }
+        }
+
+        private void HandleInvoiceWithBalancePayments(Invoice inv, Payment payment, ref string responseMessage)
+        {
+            decimal totalPayments = inv.Payments.Sum(x => x.Amount);
+            if (totalPayments != 0 && inv.Amount == totalPayments)
+            {
+                responseMessage = "invoice was already fully paid";
+            }
+            else if (totalPayments != 0 && payment.Amount > (inv.Amount - inv.AmountPaid))
+            {
+                responseMessage = "the payment is greater than the partial amount remaining";
+            }
+            else
+            {
+                HandleInvoiceWithRemainingBalance(inv, payment, ref responseMessage);
+            }
+        }
+
+        private void HandleInvoiceWithRemainingBalance(Invoice inv, Payment payment, ref string responseMessage)
+        {
+            if ((inv.Amount - inv.AmountPaid) == payment.Amount)
+            {
+                ProcessPaymentForPartiallyPaidInvoice(inv, payment, ref responseMessage, "final partial payment received, invoice is now fully paid");
+            }
+            else
+            {
+                ProcessPaymentForPartiallyPaidInvoice(inv, payment, ref responseMessage, "another partial payment received, still not fully paid");
+            }
+        }
+
+        private void HandleNonPaidInvoice(Invoice inv, Payment payment, ref string responseMessage)
+        {
+            if (payment.Amount > inv.Amount)
+            {
+                responseMessage = "the payment is greater than the invoice amount";
+            }
+            else if (inv.Amount == payment.Amount)
+            {
+                ProcessPaymentForNonPaidInvoice(inv, payment, ref responseMessage, "invoice is now fully paid");
+            }
+            else
+            {
+                ProcessPaymentForNonPaidInvoice(inv, payment, ref responseMessage, "invoice is now partially paid");
+            }
+        }
+
+        private void ProcessPaymentForPartiallyPaidInvoice(Invoice inv, Payment payment, ref string responseMessage, string responseMessageContent)
+        {
+            switch (inv.Type)
+            {
+                case InvoiceType.Standard:
+                    inv.AmountPaid += payment.Amount;
+                    inv.Payments.Add(payment);
+                    responseMessage = responseMessageContent;
+                    break;
+                case InvoiceType.Commercial:
+                    inv.AmountPaid += payment.Amount;
+                    inv.TaxAmount += payment.Amount * 0.14m;
+                    inv.Payments.Add(payment);
+                    responseMessage = responseMessageContent;
+                    break;
+                default:
+                    responseMessage = "invoice type not recognized";
+                    break;
+            }
+        }
+
+        private void ProcessPaymentForNonPaidInvoice(Invoice inv, Payment payment, ref string responseMessage, string responseMessageContent)
+        {
+            switch (inv.Type)
+            {
+                case InvoiceType.Standard:
+                case InvoiceType.Commercial:
+                    inv.AmountPaid = payment.Amount;
+                    inv.TaxAmount = payment.Amount * 0.14m;
+                    inv.Payments.Add(payment);
+                    responseMessage = responseMessageContent;
+                    break;
+                default:
+                    responseMessage = "invoice type not recognized";
+                    break;
+            }
+        }
+
+    }
+}
diff --git a/RefactorThis.Persistence/IRepository/IInvoiceRepository.cs b/RefactorThis.Persistence/IRepository/IInvoiceRepository.cs
new file mode 100644
index 0000000..f6a727b
--- /dev/null
+++ b/RefactorThis.Persistence/IRepository/IInvoiceRepository.cs
@@ -0,0 +1,16 @@
+using RefactorThis.Persistence.Models;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace RefactorThis.Persistence.IRepository
+{
+    public interface IInvoiceRepository
+    {
+        Invoice GetInvoice(string reference);
+        void SaveInvoice(Invoice invoice);
+        void Add(Invoice invoice);
+    }
+}
diff --git a/RefactorThis.Persistence/Invoice.cs b/RefactorThis.Persistence/Invoice.cs
deleted file mode 100644
index bd4370d..0000000
--- a/RefactorThis.Persistence/Invoice.cs
+++ /dev/null
@@ -1,31 +0,0 @@
-using System.Collections.Generic;
-
-namespace RefactorThis.Persistence
-{
-	public class Invoice
-	{
-		private readonly InvoiceRepository _repository;
-		public Invoice( InvoiceRepository repository )
-		{
-			_repository = repository;
-		}
-
-		public void Save( )
-		{
-			_repository.SaveInvoice( this );
-		}
-
-		public decimal Amount { get; set; }
-		public decimal AmountPaid { get; set; }
-		public decimal TaxAmount { get; set; }
-		public List<Payment> Payments { get; set; }
-		
-		public InvoiceType Type { get; set; }
-	}
-
-	public enum InvoiceType
-	{
-		Standard,
-		Commercial
-	}
-}
\ No newline at end of file
diff --git a/RefactorThis.Persistence/InvoiceRepository.cs b/RefactorThis.Persistence/InvoiceRepository.cs
deleted file mode 100644
index 38548c7..0000000
--- a/RefactorThis.Persistence/InvoiceRepository.cs
+++ /dev/null
@@ -1,21 +0,0 @@
-namespace RefactorThis.Persistence {
-	public class InvoiceRepository
-	{
-		private Invoice _invoice;
-
-		public Invoice GetInvoice( string reference )
-		{
-			return _invoice;
-		}
-
-		public void SaveInvoice( Invoice invoice )
-		{
-			//saves the invoice to the database
-		}
-
-		public void Add( Invoice invoice )
-		{
-			_invoice = invoice;
-		}
-	}
-}
\ No newline at end of file
diff --git a/RefactorThis.Persistence/Models/Invoice.cs b/RefactorThis.Persistence/Models/Invoice.cs
new file mode 100644
index 0000000..3ac2ce6
--- /dev/null
+++ b/RefactorThis.Persistence/Models/Invoice.cs
@@ -0,0 +1,24 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace RefactorThis.Persistence.Models
+{
+    public class Invoice
+    {
+        public decimal Amount { get; set; }
+        public decimal AmountPaid { get; set; }
+        public decimal TaxAmount { get; set; }
+        public List<Payment> Payments { get; set; }
+
+        public InvoiceType Type { get; set; }
+    }
+
+    public enum InvoiceType
+    {
+        Standard,
+        Commercial
+    }
+}
diff --git a/RefactorThis.Persistence/Models/Payment.cs b/RefactorThis.Persistence/Models/Payment.cs
new file mode 100644
index 0000000..5c63d5a
--- /dev/null
+++ b/RefactorThis.Persistence/Models/Payment.cs
@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace RefactorThis.Persistence.Models
+{
+    public class Payment
+    {
+        public decimal Amount { get; set; }
+        public string Reference { get; set; }
+    }
+}
diff --git a/RefactorThis.Persistence/Payment.cs b/RefactorThis.Persistence/Payment.cs
deleted file mode 100644
index db29372..0000000
--- a/RefactorThis.Persistence/Payment.cs
+++ /dev/null
@@ -1,8 +0,0 @@
-namespace RefactorThis.Persistence
-{
-	public class Payment
-	{
-		public decimal Amount { get; set; }
-		public string Reference { get; set; }
-	}
-}
\ No newline at end of file
diff --git a/RefactorThis.Persistence/Repository/InvoiceRepository.cs b/RefactorThis.Persistence/Repository/InvoiceRepository.cs
new file mode 100644
index 0000000..0073738
--- /dev/null
+++ b/RefactorThis.Persistence/Repository/InvoiceRepository.cs
@@ -0,0 +1,30 @@
+using RefactorThis.Persistence.IRepository;
+using RefactorThis.Persistence.Models;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace RefactorThis.Persistence.Repository
+{
+    public class InvoiceRepository : IInvoiceRepository
+    {
+        private Invoice _invoice;
+
+        public Invoice GetInvoice(string reference)
+        {
+            return _invoice;
+        }
+
+        public void SaveInvoice(Invoice invoice)
+        {
+            //saves the invoice to the database
+        }
+
+        public void Add(Invoice invoice)
+        {
+            _invoice = invoice;
+        }
+    }
+}