Dear :

Thank you for your letter of [date]. You are correct that we made an error on our invoice No. . We unintentionally overlooked the percent discount that you are entitled to. A corrected invoice will be sent to you today.

We are sorry for the inconvenience this mistake caused you and we assure you that we will make special measures to prevent any recurrence of similar billing errors.

Sincerely yours,

[name]

[title]