Write a letter seeking apology for not crediting payment.

Dear Sir,

This is to convey our thanks for your assistance in helping us to locate your payment of (date) which had not been credited to your account. Kindly accept our apology for this error and for the overdue notices that were sent to you.

We highly appreciate your patronage and are regret the inconvenience caused by this error.

 

Thanking you.

 
Yours truly,