2. Please use a specific date rather than "a few days ago", which is vague.
3. It is better to state what the mistake was at the beginning of the body paragraph, since it was an obvious mistake proved by clear evidence.
4. Rather than simply saying you are expecting the reader's reply you can express your confidence that this error can be corrected and that the bank will honour your request for a refund without delay.
5. Always use full name on the signature block of business letters.
6. Enclose your evidence like the account record of your payment for both the overdrawn and the charged amount.