At the end of a sales letter, state the course of action:
The last paragraph of a sales letter generally should end with the writer spelling out what steps he plans to take next -- or telling the reader how to respond immediately. Without it, the reader is left with information, but not motivation.
P.S. Did you know that 80 percent of readers -- even if they're just skimming a letter -- will read the P.S.?
So unless you're writing a very formal letter, try to work in a P.S. that adds one more compelling reason for the prospect to buy.