Dear

Thank you for your letter and the photostat 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.