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.This memorandum serves as notification of an overpayment of wages that you received and the subsequent repayment that is your responsibility. The salary overpayment resulted because . We have identified the following overpayment and are in good faith voluntarily refunding all monies collected in error. How do you write a letter to inform of a double payment? Any questions or concerns regarding your current tax bill payment must be directed to your local treasurer's office.