I am writing to apologize for the error in your bill. I understand how frustrating it must be to receive an incorrect bill, and I sincerely apologize for the inconvenience this has caused.
The error occurred due to a mistake in our system, and we are currently working to rectify the issue. We take our responsibility to provide accurate bills to our customers very seriously, and we are doing everything we can to ensure that this does not happen again.
We understand that this has caused you a great deal of frustration, and we would like to make it up to you. We are offering you a [discount/credit/etc.] of [amount] on your next bill. We hope that this will help to make up for the inconvenience.
We apologize again for the error in your bill. We are committed to providing our customers with accurate bills, and we are doing everything we can to make sure this does not happen again. If you have any further questions or concerns, please do not hesitate to contact us.