Ability to Apply a Customer Payment Entirely to Customer's Credit Balance with Reference Number

Some customers continue to overpay and ask us to put the payment as a Credit Balance on their account. What if there is no "open" invoice and you would like to add the payment? Or you would like to keep the credit on file with a reference to their check number (and show the payment)? Currently, you have to select an invoice, then apply $0.00 as the amount. Is there a better way to handle these.