I do not visit this site very often.....but thought I would comment simply because I have been using direct mail marketing for all my biz..ess since the stone age!
Don't get too caught up in the details of the letter/card. The average motivated person is only going to spend two tiny seconds MAXIMUM looking at it anyway. They see what it is for (buy my house) and then look for contact information.
Which means....the most important part of any direct mail (any kind) is how you respond, communicate and sale when they call you......
So, you can have the most technically correct guru version of a letter, but if you don't know how to listen, ask the right questions and lead the conversation all the letter writing has just an accedemic exercise.