Tests show the P.S. is one of the most widely and first read elements in a letter—B-to-B and consumer. In fact, 30 percent-plus look at the P.S. first. AT&T takes advantage of this with their new customer signup thank you letter.
That's why when I write a letter, I write the entire letter first, then pull out a key benefit and move it to the P.S. where I know it will get seen first.