Write a letter demanding payment.

Dear sir/ Madam

This is to express our understanding as how exasperating the matter 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.

We have been happy with our association with you as you have been our valued customer for a long time and we appreciate your affording us the opportunity to serve you. You may rest assured that this problem will not recur again.

Thanking You


Yours truly,