diff --git a/RefactorThis.Domain/InvoiceService.cs b/RefactorThis.Domain/InvoiceService.cs
index fbd674c..714ebfa 100644
--- a/RefactorThis.Domain/InvoiceService.cs
+++ b/RefactorThis.Domain/InvoiceService.cs
@@ -4,146 +4,74 @@
 
 namespace RefactorThis.Domain
 {
-	public class InvoiceService
-	{
-		private readonly InvoiceRepository _invoiceRepository;
+    public class InvoiceService
+    {
+        private readonly InvoiceRepository _invoiceRepository;
 
-		public InvoiceService( InvoiceRepository invoiceRepository )
-		{
-			_invoiceRepository = invoiceRepository;
-		}
+        public InvoiceService(InvoiceRepository invoiceRepository)
+        {
+            _invoiceRepository = invoiceRepository;
+        }
 
-		public string ProcessPayment( Payment payment )
-		{
-			var inv = _invoiceRepository.GetInvoice( payment.Reference );
+        public string ProcessPayment(Payment payment)
+        {
+            var invoice = _invoiceRepository.GetInvoice(payment.Reference);
 
-			var responseMessage = string.Empty;
+            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();
+            if (invoice == null)
+            {
+                throw new InvalidOperationException("There is no invoice matching this payment");
+            }
+            else
+            {
+                if (invoice.Amount == 0)
+                {
+                    if (!invoice.InvoiceHasPayments)
+                    {
+                        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 (invoice.InvoiceHasPayments)
+                    {
+                        if (invoice.InvoiceIsAlreadyFullyPaid)
+                        {
+                            responseMessage = "invoice was already fully paid";
+                        }
+                        else if (invoice.IsPaymentsGreaterThanThePartial(payment))
+                        {
+                            responseMessage = "the payment is greater than the partial amount remaining";
+                        }
+                        else
+                        {
+                            responseMessage = invoice.PartialPay(payment);
 
-			return responseMessage;
-		}
-	}
+                        }
+                    }
+                    else
+                    {
+                        if (payment.Amount > invoice.Amount)
+                        {
+                            responseMessage = "the payment is greater than the invoice amount";
+                        }
+                        else
+                        {
+                            responseMessage = invoice.FullPay(payment);
+
+                        }
+                    }
+                }
+            }
+
+            invoice.Save();
+
+            return responseMessage;
+        }
+    }
 }
\ No newline at end of file
diff --git a/RefactorThis.Persistence/Invoice.cs b/RefactorThis.Persistence/Invoice.cs
index bd4370d..6e112fc 100644
--- a/RefactorThis.Persistence/Invoice.cs
+++ b/RefactorThis.Persistence/Invoice.cs
@@ -1,31 +1,83 @@
+using System;
 using System.Collections.Generic;
+using System.Linq;
 
 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
-	}
+    public class Invoice
+    {
+        private readonly decimal _taxPercentage = 0.14m;
+        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 bool InvoiceHasPayments { get => Payments != null && Payments.Any(); }
+        public bool InvoiceIsAlreadyFullyPaid { get => Payments.Sum(x => x.Amount) != 0 && Amount == Payments.Sum(x => x.Amount); }
+        public bool IsPaymentsGreaterThanThePartial(Payment payment) => Payments.Sum(x => x.Amount) != 0 && payment.Amount > (Amount - AmountPaid);
+        public string PartialPay(Payment payment)
+        {
+            if (!Type.IsInvoiceTypeSupported())
+            {
+                throw new ArgumentOutOfRangeException();
+            }
+            bool isPaymentFinalPartial = (Amount - AmountPaid) == payment.Amount;
+
+            string responseMessage = isPaymentFinalPartial ? "final partial payment received, invoice is now fully paid" : "another partial payment received, still not fully paid";
+
+            if (Type == InvoiceType.Commercial)
+            {
+                TaxAmount += payment.Amount * _taxPercentage;
+            }
+
+            AmountPaid += payment.Amount;
+            Payments.Add(payment);
+
+            return responseMessage;
+        }
+
+        public string FullPay(Payment payment)
+        {
+            if (!Type.IsInvoiceTypeSupported())
+            {
+                throw new ArgumentOutOfRangeException();
+            }
+
+            bool isFullPayment = Amount == payment.Amount;
+            string responseMessage = isFullPayment ? "invoice is now fully paid" : "invoice is now partially paid";
+
+            AmountPaid = payment.Amount;
+            TaxAmount = payment.Amount * _taxPercentage;
+            Payments.Add(payment);
+
+            return responseMessage;
+        }
+    }
+
+    public enum InvoiceType
+    {
+        Standard,
+        Commercial
+    }
+
+    public static class InvoiceTypeExtension
+    {
+        public static bool IsInvoiceTypeSupported(this InvoiceType invoiceType)
+        {
+            return invoiceType == InvoiceType.Standard || invoiceType == InvoiceType.Commercial;
+        }
+    }
+
 }
\ No newline at end of file