Dear _________________,
Your final loan payment was received, and I wanted to take this opportunity to thank you for the responsible way in which you handled your account.
It was a pleasure assisting you with your personal finances. We are proud of our service and hope you found it satisfactory in every way. If so, perhaps you will recommend _______________Bank to your friends.
Be sure to let us know if there is any other way in which we can assist you. If you'll call to let me know you are coming to the bank, I'll be glad to have whatever you need ready when you arrive. You have my word on it!
Sincerely,