[DATE, ex. Monday, March 31, 2008]

[NAME, COMPANY AND ADDRESS, ex.
Link Building
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245]

Dear [NAME, ex. Link Building],

Since we value your business greatly, I have been inspecting our invoices to ensure that we have made no errors.  Unfortunately, however, I have uncovered an error on invoice #[STATE INVOICE NUMBER].  Please accept my apologies for over-billing you by $[AMOUNT].

I will immediately nullify that invoice ( [STATE INVOICE NUMBER, ex. #A423F] ) and issue you a correct one, as follows:

[LIST NEW INVOICE DETAILS, ex.
            5 Magnaflux compressors @ $4,000                    $20,000
            Tax @ 5%                                                       1,000
            Total                                                            $21,000]

Let me assure you that I will personally inspect our invoices to you in the future to ensure that there will be no further errors.  Feel free to contact me at XXX-XXXX if there are any other issues you wish me to address.

Sincerely,

[YOUR NAME, ex. Jill Jones]