Your Name
Your Address
Your City, State, Zip Code

Date

Their Name
Their Address
Their City, State, Zip Code

Dear Recipient Name,

This month's payment of $________  on invoice number ________________ was due on _____________.   Unfortunately, this payment was mailed today due to an oversight on our part.   We apologize for this mistake and we will take care to ensure this does not happen again.   Feel free to call me if you have any further questions at _________________.

Sincerely,

Your Signature