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.A statement or letter from the company indicating: (1) Property Index Number, (2) taxpayer's name,. Our document template is designed to make the process of writing an "Inform a Customer of an Overpayment" letter quick, easy, and stress-free. Click here to search for any overpayments on your PIN. The client will have 30 days from the date of the Overpayment letter to submit the request and supporting documents.