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.The salary overpayment resulted because . Send sample overpayment letter to customer via email, link, or fax. You can also download it, export it or print it out. This memorandum serves as notification of an overpayment of wages that you received and the subsequent repayment that is your responsibility.