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 . The IRS uses Letter 3286C to inform you that they believe you may have overpaid for a particular year and will refund you the difference. Type a letter to the person who made the payment, explaining that he paid in excess of the amount due. We have identified the following overpayment and are in good faith voluntarily refunding all monies collected in error.