What did your letters look like? Often, if you use labels or a postage meter people automatically think that it is junk mail. I have found from experience that that a plain envelope (no logo) with a hand written address and stamp get the best results.
I have not tried a direct mailer for radon, but have for other types of services such as a one year warranty inspection.
Your question about the postcard - Postcards are many time more effective in getting people to read the information for the simple reason of not having to open an envelope, just turn the postcard over and there you go. The only problem with postcards is the cost of having them printed vs. printing and mailing your own (good quality of course) flyer’s. You may save some money on postage .26 vs. .41 but you will have to do the math so see if it is worth it.
The only way to really know if a marketing idea is going to work is to test it and track it.
Hope that helped (some).