A vendor offers a 1% discount if an invoice is paid within 10 days, but only on the non-freight portion of the invoice.

So if an invoice is $100, and freight is $5, the discount should be $1.00, not $1.05.

My client would like to be able to edit the discount base when entering the invoice, but unlike the due date, discount date, and discount rate, the discount base does not seem to be editable. Editing the discount base after the invoice is posted is too unwieldy.

Is there a way to accomplish this?

Steve