I'm not 100% sure this works or is very clean, certainly not than a more technical product solution, BUT you could create a new invoice that uses line items for each invoice total and connects to a liability account similar to a retainer... but a backwards retainer paid after. In any case, the customer sees one invoice, pays one invoice, that covers all of the outstanding invoices. Now you have this "retainer" balance. Then go to all the outstanding invoices covered and add a line that zeros the balance out with a negative balance retainer line item. So each invoice still shows all the detail but is zero because a retainer was applied. Just an idea.