Simply replace the unfilled areas such as "" and "[date]" with your information.
It is with my sincerest apologies that I write to you. I have thoroughly examined our records and you were correct that the past due notices sent to you were in error. The error has been corrected and you current account balance is $[amount].
Please give me an opportunity to explain. Your payment was received in a timely manner. However, due to an error it was [brief explanation of error]. Therefore, you began receiving our routine notices requesting payment. After the payment period had passed, our [system/ credit department] automatically generates Past Due notices. Please be assured that this problem has been rectified.
I am sorry that you have had this experience. Please be assured that we are implementing measures to prevent this from happening again. You have always been a very loyal customer and we appreciate your business.
Please let me know if there is anything else I can do for you. We look forward to serving you in the future.