Show balance due on invoices

Sometimes we have a client send a cheque for payment of an invoice even though it was paid through PayPal.  I realize the payment method shows on the invoice but they pay anyways and then we spend time fixing the issue.

Other times they use a Purchase Order and then send us the cheque.  Now they want a receipt or invoice showing a balance due as $0.00.

If the invoice had a field that show Balance Due based on the the Invoice Status of Paid.