You've followed my advice and kept your sales letter short and to the point. But you want a few words in your letter to stand out. Old Advice: Before the advent of computers with full-featured word processors and a laser printer on just about everyone's desk, I would have recommended underscoring the important words. If you did anything fancier, you shouted to your readers that you were sending out a mass mailing.
New Advice: Nowadays, however, bold or italics type is a fine way to make your point stand out and still look like a personal letter. You can even switch to a completely different font or typestyle, or increase the size of your font. All this is now acceptable in personal business correspondence because the technology is available to just about everybody. The only thing I don't recommend is using reverse type (white letters on a black background). It just doesn't look right on a business letter.