Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Mr./Ms./Mrs.] [last name of customer]
A review of our files has indicated that your credit card was charged in error on [date] for [amount of charge]. We would like to apologize for this mistake and for any inconvenience that this has caused.
Please know that we take the utmost care with our customer accounts, and are investigating this issue. We assure you that it will not happen again, and we have credited your card for the amount charged in error.
Thank you for you continued business throughout the years, and we hope to be able to continue to serve you. Please don't hesitate to contact us with any concerns or questions.