Hi,
I am interested in sending some thank you letters for customers who use our courier service. Can You Help By Providing A Draft?
Thank You

I certainly don't want to write one for you. They're relatively simple, just get a standard professional letter format and make sure they're individualized. I find that comes across much better than a general letter.

Thank them for their continued support and offer a deal... that will get them coming back for more. ;)