Ive added shipping by weight to a website however it seems that the original price is overriding it?

Im using the code below to work out shipping using "price per kg * weight in kg" I then add the cost of the selected shipping method to that and set order.shippingFeeWithoutVAT to the new value.

When I go through the checkout with a shipping method cost of £0, the shipping by weight kicks in and shows £106 in the log which is what it should be, but in the checkout it still shows £0, however if I do the same but with a shipping method cost of £5, the shipping by weight kicks in and shows £111 which is also correct. Why would it be setting my shipping cost to £0 even after I have set it to £111?

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Web;
using System.Xml.XPath;
using TeaCommerce.Data;
using TeaCommerce.Data.Extensibility;
using TeaCommerce.Data.Tools;

/// <summary>
/// Summary description for TeaCommerceExtension
/// </summary>
namespace TeaCommerce.WebShop.Integration
{
    public class TeaCommerceExtension : ITeaCommerceExtension
    {
        #region ITeaCommerceExtension Members

        public void Initialize()
        {
            WebshopEvents.OrderLineChanged += WebshopEvents_OrderLineChanged;
            WebshopEvents.ShippingMethodChanged += WebshopEvents_ShippingMethodChanged;
            WebshopEvents.CurrencyChanged += WebshopEvents_CurrencyChanged;
            WebshopEvents.OrderLineRemoved += WebshopEvents_OrderLineRemoved;
        }

        void WebshopEvents_OrderLineRemoved(Order order, OrderLine orderLine)
        {
            UpdateOrderShippingCost(order);
        }

        /// <summary>
        /// On currency change
        /// </summary>
        /// <param name="order"></param>
        /// <param name="currency"></param>
        void WebshopEvents_CurrencyChanged(Order order, Currency currency)
        {
            UpdateOrderShippingCost(order);
        }

        /// <summary>
        /// On shipping method change
        /// </summary>
        /// <param name="order"></param>
        /// <param name="shippingMethod"></param>
        void WebshopEvents_ShippingMethodChanged(Order order, ShippingMethod shippingMethod)
        {
            UpdateOrderShippingCost(order);
        }

        void WebshopEvents_OrderLineChanged(Order order, OrderLine orderLine)
        {
            UpdateOrderShippingCost(order);
        }

        #endregion

        Boolean debug = true;

        private void UpdateOrderShippingCost(Order order)
        {
            XPathNodeIterator allXml = umbraco.library.GetXmlAll();

            XPathNavigator shippingRules = allXml.Current.SelectSingleNode("//ShippingRules");
            decimal defaultWeightInKg = (decimal)PriceHelper.ParsePrice(shippingRules.SelectSingleNode("defaultWeight").Value);
            decimal defaultPricePerKg = (decimal)PriceHelper.ParsePrice(shippingRules.SelectSingleNode("defaultPricePerKG").Value);

            // Log a, error in the database
            if (debug)
            {
                umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                    umbraco.BusinessLogic.User.GetUser(0), -1,
                    "default weight: " + defaultWeightInKg + " and default price per kg: " + defaultPricePerKg);
            }

            decimal totalWeightInKg = 0;
            foreach (OrderLine ol in order.OrderLines)
            {
                OrderLineProperty weightProp = ol.Properties.FirstOrDefault(p => p.Alias == "productWeight");

                if (debug)
                {
                    foreach (OrderLineProperty op in ol.Properties)
                    {
                        // Log a, error in the database
                        umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                            umbraco.BusinessLogic.User.GetUser(0), -1,
                            "order property " + op.Alias + " is " + op.Value.ToString());
                    }
                }

                if (weightProp != null && !string.IsNullOrEmpty(weightProp.Value))
                {
                    totalWeightInKg += (decimal)PriceHelper.ParsePrice(weightProp.Value.ToString()) * ol.Quantity;
                    if (debug)
                    {
                        // Log a, error in the database
                        umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                            umbraco.BusinessLogic.User.GetUser(0), -1,
                            "reached product weight");
                    }
                }
                else
                {
                    totalWeightInKg += defaultWeightInKg * ol.Quantity;
                }
            }

            if (debug)
            {
                // Log a, error in the database
                umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                    umbraco.BusinessLogic.User.GetUser(0), -1,
                    "total weight in kg: " + totalWeightInKg);
            }

            XPathExpression weightPathXPath = XPathExpression.Compile("//ShippingRule");
            weightPathXPath.AddSort("upToWeight", XmlSortOrder.Ascending, XmlCaseOrder.None, string.Empty, XmlDataType.Number);
            XPathNodeIterator weightRules = shippingRules.Select(weightPathXPath);
            decimal shippingPricePerKgWithoutVAT = defaultPricePerKg;
            while (weightRules.MoveNext())
            {
                if (debug)
                {
                    // Log a, error in the database
                    umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                        umbraco.BusinessLogic.User.GetUser(0), -1,
                        "found rule: " + weightRules.Current.SelectSingleNode("upToWeight").Value);
                }

                if (int.Parse(weightRules.Current.SelectSingleNode("upToWeight").Value) >= totalWeightInKg)
                {
                    shippingPricePerKgWithoutVAT = (decimal)PriceHelper.ParsePrice(weightRules.Current.SelectSingleNode("shippingPrice").Value);
                    if (debug)
                    {
                        // Log a, error in the database
                        umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                            umbraco.BusinessLogic.User.GetUser(0), -1,
                            "match: " + weightRules.Current.SelectSingleNode("upToWeight").Value);
                    }
                    break;
                }
            };

            if (debug)
            {
                // Log a, error in the database
                umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                    umbraco.BusinessLogic.User.GetUser(0), -1,
                    "shipping price per kg: " + shippingPricePerKgWithoutVAT);
            }

            decimal totalShippingPriceWithoutVAT = shippingPricePerKgWithoutVAT * totalWeightInKg;

            if (debug)
            {
                // Log a, error in the database
                umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                    umbraco.BusinessLogic.User.GetUser(0), -1,
                    "total Shipping Price Without VAT: " + totalShippingPriceWithoutVAT);
            }

            OrderProperty shippingPriceOP = order.Properties.FirstOrDefault(p => p.Alias == "shippingPriceWithoutVAT" + order.Currency.ISOCode);
            OrderProperty totalWeightInKgProp = order.Properties.FirstOrDefault(p => p.Alias == "totalWeightInKg");
            if (shippingPriceOP == null)
            {
                shippingPriceOP = new OrderProperty("shippingPriceWithoutVAT" + order.Currency.ISOCode, totalShippingPriceWithoutVAT.ToString());
                order.AddProperty(shippingPriceOP);
                totalWeightInKgProp = new OrderProperty("totalWeightInKg", totalWeightInKg.ToString());
                order.AddProperty(totalWeightInKgProp);
            }
            else
            {
                if (Decimal.Parse(shippingPriceOP.Value) != totalShippingPriceWithoutVAT)
                    shippingPriceOP.Value = totalShippingPriceWithoutVAT.ToString();

                if (Decimal.Parse(totalWeightInKgProp.Value) != totalWeightInKg)
                    totalWeightInKgProp.Value = totalWeightInKg.ToString();
            }

            if (debug)
            {
                // Log a, error in the database
                umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                    umbraco.BusinessLogic.User.GetUser(0), -1,
                    "shipping method cost is: " + order.ShippingMethod.GetFeeWithoutVAT(order));
            }

            order.ShippingFeeWithoutVAT = order.ShippingMethod.GetFeeWithoutVAT(order) + totalShippingPriceWithoutVAT;

            if (debug)
            {
                // Log a, error in the database
                umbraco.BusinessLogic.Log.Add(umbraco.BusinessLogic.LogTypes.Error,
                    umbraco.BusinessLogic.User.GetUser(0), -1,
                    "final value added to order: " + order.ShippingFeeWithoutVAT);
            }

            //OrderProperty shippingPriceOP = order.Properties.FirstOrDefault(p => p.Alias == "shippingPriceWithoutVAT" + order.Currency.ISOCode);
            //OrderProperty totalWeightInKgProp = order.Properties.FirstOrDefault(p => p.Alias == "totalWeightInKg");
            //if (shippingPriceOP == null)
            //{
            //    shippingPriceOP = new OrderProperty("shippingPriceWithoutVAT" + order.Currency.ISOCode, totalShippingPriceWithoutVAT.ToString());
            //    order.AddProperty(shippingPriceOP);
            //    totalWeightInKgProp = new OrderProperty("totalWeightInKg", totalWeightInKg.ToString());
            //    order.AddProperty(totalWeightInKgProp);
            //}
            //else
            //{
            //    if (Decimal.Parse(shippingPriceOP.Value) != totalShippingPriceWithoutVAT)
            //        shippingPriceOP.Value = totalShippingPriceWithoutVAT.ToString();

            //    if (Decimal.Parse(totalWeightInKgProp.Value) != totalWeightInKg)
            //        totalWeightInKgProp.Value = totalWeightInKg.ToString();
            //}
            //if (order.ShippingMethod.Id == 2 && order.ShippingFeeWithoutVAT != totalShippingPriceWithoutVAT)
            //    order.ShippingFeeWithoutVAT = totalShippingPriceWithoutVAT;

            order.Save();
        }
    }
}

Thanks in advance,
Luke