[DATE, ex. Wednesday, June 11, 1998]

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

Dear [NAME, ex. John Smith],

Thank you for your [STATE FORM OF COMMUNICATION, ex. telephone call yesterday], which prompted me to check our records.  You are absolutely correct. 

In error, we have over-billed you by $[AMOUNT] on invoice #[ INVOICE NUMBER].  [STATE NATURE OF ERROR, ex. Instead of charging you for the four units you ordered and were shipped to you, we charged you for five.]  Please accept my apologies.

I have immediately nullified the incorrect invoice ([INVOICE NUMBER, ex. #A423F]) and issued you a new one as follows: 

[LIST NEW INVOICE DETAILS, ex.

          4 Magnaflux compressors @ $5,000                   $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.  {STATE INCENTIVE, IN ANY, ex. Since I feel embarrassed over the matter, I will offer you a 5% discount on your next order.}  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]