Apology letter to Customer for Billing Error.
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