You deserve an explanation for what went wrong in our accounting department, and I hope that this letter will serve to resolve our recent difficulties. I know that you can appreciate the fact that it has taken some time to find out exactly what occurred, and, therefore, please accept our apologies for the delay in this response. Apparently, your payment was received in a timely fashion, but it was credited to an account which bears a similar name to yours.
Therefore, we commenced sending you our standard notices requesting payment, in keeping with our routine policy. Even after the posting error was rectified, our accounting department failed to notify our credit department, which is why you continued to receive our correspondence demanding payment. I know how exasperating this has been for you and I am deeply sorry that it has taken so long to straighten out this problem. While there is a procedure within our firm to preclude this type of error from occurring, we are reinforcing this procedure. You have been a valued customer of ours for a long time and we appreciate your affording us the opportunity to serve you.
You may rest assured that this problem will not surface again. Thank you, Yours Truly,(Your Name)