Please allow for the addition of line items to percentage payment invoices, even after payment(s) have been submitted.

For example, I require 80% down payment for products, with the remaining 20% balance due before passing the products onto the client. To pass on the client, I need to ship the products. The shipping costs are TBD at the time of the initial invoice and need to be added to the balance due.

If I change the invoice to a draft to add the shipping cost to the balance due invoice, it only charges 20%, and it doesn't generate another invoice for the remaining 80% balance for this additional line item. I then have to create ANOTHER invoice for the 100% of the 80% balance. I want to avoid the additional invoices.