Hey Jim,
Welcome to the party.
Three things...
1. As I read the letter, the main question I had in my mind is, "what's the catch?"
What's in it for you? How can you manage to do this when all the advertising I've ever tried for my home improvement contracting biz fell flat on its face?
What's the catch? Be upfront... transparent... THEN you can wine and dine me
2. Also, if the priority mail got you a response boost, why not try mailing in a fake priority envelope from someone like
Envelopes.com -- Direct Mail Services, Envelopes and More! -- Home
3. What are the next steps in your sequence AFTER they get the initial letter?