You already know this, but you might have to send several letter/postcard to get a response. Just typical sales strategy.
Not sure if I saw in on this site or elsewhere…throw some hard candy in the envelope. Makes it non-flat and increases the odds of it getting opened.
Another thing. Add a piece of cardboard to the envelope (8x10) and write large on the outside “Picture of your XX state house enclosed”
A picture of the house, a picture of a check. Ask him to call you to fill in the check amount. Tell him BOTH are accurate pictures of his house…'cause they are.