[DATE, ex. Wednesday, June 11, 1998]
[NAME, COMPANY AND ADDRESS, ex.
1234 First Street
Anycity, Anystate 85245]
Dear [NAME, ex. John Smith],
Allow me to apologize for the incorrect payment we made to you regarding your invoice #[INVOICE NUMBER]. Our check #[CHECK NUMBER] for $[AMOUNT] is short of the total amount due, $[ AMOUNT].
Enclosed, please find our check #[CHECK NUMBER] for $[AMOUNT], representing the difference between $[AMOUNT] and [AMOUNT].
Please accept my apologies and assurance that this will not occur again.
[YOUR NAME, ex. Jill Jones]