diff --git a/RefactorThis.Domain.Tests/InvoicePaymentProcessorTests.cs b/RefactorThis.Domain.Tests/InvoicePaymentProcessorTests.cs
index 3a607fd..c070954 100644
--- a/RefactorThis.Domain.Tests/InvoicePaymentProcessorTests.cs
+++ b/RefactorThis.Domain.Tests/InvoicePaymentProcessorTests.cs
@@ -9,7 +9,7 @@ namespace RefactorThis.Domain.Tests
 	public class InvoicePaymentProcessorTests
 	{
 		[Test]
-		public void ProcessPayment_Should_ThrowException_When_NoInoiceFoundForPaymentReference( )
+		public void ProcessPayment_Should_ThrowException_When_NoInvoiceFoundForPaymentReference( )
 		{
 			var repo = new InvoiceRepository( );
 
@@ -142,6 +142,7 @@ public void ProcessPayment_Should_ReturnFullyPaidMessage_When_PartialPaymentExis
 			var repo = new InvoiceRepository( );
 			var invoice = new Invoice( repo )
 			{
+				Type = InvoiceType.Standard,
 				Amount = 10,
 				AmountPaid = 5,
 				Payments = new List<Payment>
@@ -164,6 +165,44 @@ public void ProcessPayment_Should_ReturnFullyPaidMessage_When_PartialPaymentExis
 			var result = paymentProcessor.ProcessPayment( payment );
 
 			Assert.AreEqual( "final partial payment received, invoice is now fully paid", result );
+			Assert.AreEqual( 10, invoice.AmountPaid );
+			Assert.AreEqual( 0m, invoice.TaxAmount );
+			Assert.AreEqual( 2, invoice.Payments.Count );
+		}
+
+		[Test]
+		public void ProcessPayment_Should_ApplyTaxToCommercialInvoice_When_PartialPaymentExistsAndAmountPaidEqualsAmountDue( )
+		{
+			var repo = new InvoiceRepository( );
+			var invoice = new Invoice( repo )
+			{
+				Type = InvoiceType.Commercial,
+				Amount = 10,
+				AmountPaid = 5,
+				TaxAmount = 0.7m,
+				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 );
+			Assert.AreEqual( 10, invoice.AmountPaid );
+			Assert.AreEqual( 1.4m, invoice.TaxAmount );
+			Assert.AreEqual( 2, invoice.Payments.Count );
 		}
 
 		[Test]
@@ -174,7 +213,7 @@ public void ProcessPayment_Should_ReturnFullyPaidMessage_When_NoPartialPaymentEx
 			{
 				Amount = 10,
 				AmountPaid = 0,
-				Payments = new List<Payment>( ) { new Payment( ) { Amount = 10 } }
+				Payments = new List<Payment>( )
 			};
 			repo.Add( invoice );
 
@@ -187,7 +226,10 @@ public void ProcessPayment_Should_ReturnFullyPaidMessage_When_NoPartialPaymentEx
 
 			var result = paymentProcessor.ProcessPayment( payment );
 
-			Assert.AreEqual( "invoice was already fully paid", result );
+			Assert.AreEqual( "invoice is now fully paid", result );
+			Assert.AreEqual( 10, invoice.AmountPaid );
+			Assert.AreEqual( 1.4m, invoice.TaxAmount );
+			Assert.AreEqual( 1, invoice.Payments.Count );
 		}
 
 		[Test]
@@ -196,6 +238,7 @@ public void ProcessPayment_Should_ReturnPartiallyPaidMessage_When_PartialPayment
 			var repo = new InvoiceRepository( );
 			var invoice = new Invoice( repo )
 			{
+				Type = InvoiceType.Commercial,
 				Amount = 10,
 				AmountPaid = 5,
 				Payments = new List<Payment>
@@ -218,6 +261,9 @@ public void ProcessPayment_Should_ReturnPartiallyPaidMessage_When_PartialPayment
 			var result = paymentProcessor.ProcessPayment( payment );
 
 			Assert.AreEqual( "another partial payment received, still not fully paid", result );
+			Assert.AreEqual( 6, invoice.AmountPaid );
+			Assert.AreEqual( 0.14m, invoice.TaxAmount );
+			Assert.AreEqual( 2, invoice.Payments.Count );
 		}
 
 		[Test]
@@ -242,6 +288,9 @@ public void ProcessPayment_Should_ReturnPartiallyPaidMessage_When_NoPartialPayme
 			var result = paymentProcessor.ProcessPayment( payment );
 
 			Assert.AreEqual( "invoice is now partially paid", result );
+			Assert.AreEqual( 1, invoice.AmountPaid );
+			Assert.AreEqual( 0.14m, invoice.TaxAmount );
+			Assert.AreEqual( 1, invoice.Payments.Count );
 		}
 	}
 }
\ No newline at end of file
diff --git a/RefactorThis.Domain/InvoiceService.cs b/RefactorThis.Domain/InvoiceService.cs
index fbd674c..f79ddb5 100644
--- a/RefactorThis.Domain/InvoiceService.cs
+++ b/RefactorThis.Domain/InvoiceService.cs
@@ -17,133 +17,84 @@ 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.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.HasPayments( ) )
 				{
-					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( );
-							}
-						}
-					}
+					throw new InvalidOperationException(
+						"The invoice is in an invalid state, it has an amount of 0 and it has payments.");
 				}
+
+				return "no payment needed";
+			}
+
+			return ( inv.HasPayments( ) )
+				? ProcessPartialPayment( inv, payment )
+				: ProcessFullPayment( inv, payment );
+		}
+
+		private static string ProcessPartialPayment( Invoice inv, Payment payment )
+		{
+			if ( inv.Amount == inv.Payments.Sum( x => x.Amount ) )
+			{
+				return "invoice was already fully paid";
+			}
+
+			if ( payment.Amount > ( inv.Amount - inv.AmountPaid ) )
+			{
+				return "the payment is greater than the partial amount remaining";
 			}
-			
-			inv.Save();
 
-			return responseMessage;
+			switch ( inv.Type )
+			{
+				case InvoiceType.Standard:
+					// TODO: Discuss with stakeholders
+					// I'm preserving existing behaviour here - but it seems wrong!
+					// I suspect that either this is supposed to be taxed - or that Standard
+					// invoices should not be taxed in the ProcessFullPayment() function either
+					inv.AddPayment( payment, false );
+					break;
+				case InvoiceType.Commercial:
+					inv.AddPayment( payment );
+					break;
+				default:
+					throw new ArgumentOutOfRangeException( );
+			}
+
+			inv.Save( );
+
+			return ( inv.AmountPaid == inv.Amount )
+				? "final partial payment received, invoice is now fully paid"
+				: "another partial payment received, still not fully paid";
+		}
+
+		private static string ProcessFullPayment( Invoice inv, Payment payment )
+		{
+			if ( payment.Amount > inv.Amount )
+			{
+				return "the payment is greater than the invoice amount";
+			}
+
+			switch ( inv.Type )
+			{
+				case InvoiceType.Standard:
+				case InvoiceType.Commercial:
+					inv.AddPayment( payment );
+					break;
+				default:
+					throw new ArgumentOutOfRangeException( );
+			}
+
+			inv.Save( );
+
+			return ( inv.Amount == payment.Amount )
+				? "invoice is now fully paid"
+				: "invoice is now partially paid";
 		}
 	}
 }
\ No newline at end of file
diff --git a/RefactorThis.Persistence/Invoice.cs b/RefactorThis.Persistence/Invoice.cs
index bd4370d..0a7514d 100644
--- a/RefactorThis.Persistence/Invoice.cs
+++ b/RefactorThis.Persistence/Invoice.cs
@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 
 namespace RefactorThis.Persistence
 {
@@ -21,6 +22,20 @@ public void Save( )
 		public List<Payment> Payments { get; set; }
 		
 		public InvoiceType Type { get; set; }
+
+		public bool HasPayments( )
+		{
+			return Payments != null && Payments.Any( );
+		}
+		public void AddPayment( Payment payment, bool isTaxable = true )
+		{
+			AmountPaid += payment.Amount;
+			if (isTaxable)
+			{
+				TaxAmount += payment.Amount * 0.14m;
+			}
+			Payments.Add( payment );
+		}
 	}
 
 	public enum InvoiceType