This is good news to most people. The sooner you notify the customer, the better. Indicate what corrective action was taken.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We have received your payment of <$amount> on invoice number <invoice number>. Because <reason for overpayment> you are due a credit of <$amount>.
<Handling of overpayment>.
If you have any questions, please feel free to call us at <phone number>.
encl: <List of enclosed items goes here>