I need to figure how to do the following in Invoices:

OUr customers provide us with the number of buys per month by product, then I calc the buys x retail rate, get a sub-total, then subtract a local taxes, then foreign withholding tax, then am left withthe Total amount due.

I have looked at the tax function, and it only seems to add tax to the sub-total. How can I apply a "discount" and then change the description from "Discount" to "Local Tax" and "Foreign Withholding Taxes"?