Have you tried creating your own ITaxCalculator?
https://world.episerver.com/documentation/developer-guides/commerce/orders/calculating-orders-intro/calculating-orders-tax-calculator/

The order handling is setup so that different Calculators has the logic that creates the totals that are saved on the PurchaseOrders, they are automatically called by episerver so you can't modify the totals yourself.

purchaseOrder.AcceptChanges();

_orderRepository.Save(purchaseOrder);

both those lines save the order, only one is needed. I think you should remove AcceptChanges() and try again

Hi Quan,

I have tried the following:

var orderReference = _orderRepository.SaveAsPurchaseOrder(cart);
var purchaseOrder = _orderRepository.Load<PurchaseOrder>(orderReference.OrderGroupId);

purchaseOrder.TaxTotal -= taxAmount;
purchaseOrder.Total -= taxAmount;

_orderRepository.Save(purchaseOrder);

Which still reverted the amount.

I also tried:

var orderReference = _orderRepository.SaveAsPurchaseOrder(cart);
var purchaseOrder = _orderRepository.Load<PurchaseOrder>(orderReference.OrderGroupId);

purchaseOrder.TaxTotal -= taxAmount;
purchaseOrder.Total -= taxAmount;

purchaseOrder.AcceptChanges();

Which seemed like it had saved the amount but when checking the order in commerce manager the amount has somehow still reverted :(.

I am going to double check if its being saved again further down the line.

Hi Quan,

That was it! Further on down the line we were saving the purchase order again using the orderrepository. I have now changed this to AcceptChanges() whichs works much better :).

Thanks!