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.Our document template is designed to make the process of writing an "Inform a Customer of an Overpayment" letter quick, easy, and stress-free. 2) MAKE A COPY OF EACH CERTIFIED MAIL RECEIPT which you received back indicating. The salary overpayment resulted because . This memorandum serves as notification of an overpayment of wages that you received and the subsequent repayment that is your responsibility. Open the overpaid invoice.