[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],

Thank you for [STATE NATURE OF CONTACT, ex. your letter of June 10, 1998].   You are absolutely correct; please allow me to apologize for issuing you an incorrect invoice.  I will immediately nullify that invoice (#[INVOICE NUMBER]) 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]