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.SAMPLE COMPLAINT LETTER. Date. Your Name Your Address City, State Zip Code Account Number (if applicable) Your Phone Number. Our document template is designed to make the process of writing an "Inform a Customer of an Overpayment" letter quick, easy, and stress-free. The overpayment on your tax return is not enough to cover all contributions you requested. How do I write an official demand letter to request the amount owed to me before I file in claims court in California? If HMRC have overpaid from previous years then in order to avoid complications you should write to them and confirm the amount you owe. Overpayments are subject to refund only if a claim is filed with the Office of Finance within one (1) year from the date of payment.