Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
Thank you for your assistance in helping us to locate your payment of [date], which had not been credited to your account.
Please accept our apology for this error and for the overdue notices we sent to you.
We appreciate your patronage and are sorry for the inconvenience caused by this error.
Sincerely,
[name]