Hello V:

If you offer them a 10% discount if they bring the letter with them or send a copy with their order, it won't matter too much how you word it.

But, I couldn't tell you how to word a thank you note.

excon

Thank them for their time in visiting and that they had some very interesting questions. That you are available to answer any additional questions and that you thank them for their interest in your product/service.

You may want to add that you will call them to further explain any details. (State a specific date and time that you are going to call.)

Stringer