Overpayments against a sale can either be refunded to the customer or converted into a customer credit which the customer can use against future invoices. For the open invoice in the Payment column, enter how much of the credit you want to apply.This brief but important letter notifies a customer that he or she has overpaid you. It can serve as a very good customer relations letter. Expand_more. King County accepts both credit and debit cards for online property tax payments. The only way to fix this is to either refund the excess payment to a gift card or allow the customer to purchase a gift card for a later use. Credit Card issuers are completely powerless to stop you from sending them more than your balance due.