I NEVER order without checking this Realtor coupon site.
A lot of great coupons for postcards and other print marketing here:http://realestate.reviewsanddeals.com/
In the hotter market last year, I had a letter than I printed on letterhead and sent first class. Got lots of responses that turned into business with that. But it was the message that worked in that market. I haven't sent it lately because it would work now.
So, in response to the question, I believe it's the message you send, rather than the format it goes out in.
Thanks for asking,