We are using uCommerce 3.0.0.12320 and made a cutom VAT calculation.

The Basket.config is modified as below

<value>${Basket.CustomTaxCalculation}</value>
<value>${Basket.CalculateVATTotal}</value>

    <component id="Basket.CustomTaxCalculation"
                   service="UCommerce.Pipelines.IPipelineTask`1[[UCommerce.EntitiesV2.PurchaseOrder, UCommerce]], UCommerce"
                   type="HarryWinston.Web.OrderTax.TaxCalculation, HarryWinston.Web.OrderTax" />
    

    <component id="Basket.CalculateVATTotal"
                   service="UCommerce.Pipelines.IPipelineTask`1[[UCommerce.EntitiesV2.PurchaseOrder, UCommerce]], UCommerce"
                type="UCommerce.Pipelines.Basket.CalculateOrderVatTotalTask, UCommerce.Pipelines" />

The issue is during the checkout process

TransactionLibrary.ExecuteBasketPipeline();
TransactionLibrary.Checkout();

Please review the stacktrace below:

"   at UCommerce.Pipelines.Pipeline`1.Execute(T subject)
   at UCommerce.Transactions.CheckoutService.Checkout(Basket basket)
   at UCommerce.Transactions.TransactionLibraryInternal.Checkout()
   at Castle.Proxies.TransactionLibraryInternalProxy.Checkout_callback()
   at Castle.Proxies.Invocations.TransactionLibraryInternal_Checkout.InvokeMethodOnTarget()
   at Castle.DynamicProxy.AbstractInvocation.Proceed()
   at UCommerce.Infrastructure.Interceptor.ExceptionLoggingInterceptor.Intercept(IInvocation invocation)
   at Castle.DynamicProxy.AbstractInvocation.Proceed()
   at Castle.Proxies.TransactionLibraryInternalProxy.Checkout()
   at UCommerce.Api.TransactionLibrary.Checkout()