Apology letter to Customer for Billing Error.

Example:

Dear [Name]

We would like to apologize for the error in your billing statement last [insert date]. We have now thoroughly checked our records and updated your account with the correct billing amount. The changes will be reflected on your next biling statement.

I hope this will resolve the problem to your satisfaction. If you have additional questions about your account, please feel free to contact [insert name] at [insert telephone number].

Thank you very much

Sincerely,

[Name]