Simply replace the unfilled areas such as "" and "[date]" with your information.
Thank you for your letter and the photocopy of your check, which showed that your account had been paid in full. While we are very sorry for the inconvenience this has caused you, the enclosure of your check enabled us to go through our records and pinpoint how this error occurred.
Please accept our apology for the letters we wrote under the assumption that this bill had not been paid. I know this insistance on our part must have been extremely frustrating for you, especially in light of the fact that you have always been a valued customer of ours and have paid your bills promptly.
Thank you for your patience and please be assured that we will do everything in our power to ensure that this type of error does not occur in the future.