A couple things . . .
You know, logistics. So we're both clear on how this will go. Getting a letter just for you should be fun—for both of us!
If you do not provide your mailing address or postage is too expensive, I will email your letter.
Unlikely, but if I am suddenly swarmed with more requests than I can keep up with, I will tell you. I am only human (which is literally the point).