Kentico usually uses the ShoppingCartInfo to generate Orders, which during that conversion process (ShoppingCartInfoProvider.SetOrder(ShoppingCart)
) it handles figuring out taxes and such.
var ShoppingCart = ShoppingCartInfoProvider.GetShoppingCartInfoFromOrder(PreviousOrderID);
ShoppingCart.OrderId = 0; // Will make the system think it's a new order
ShoppingCartInfoProvider.SetOrder(ShoppingCart); // Create order
That should hook up automatically the tax services to find the proper taxes and such. Try that!