That's when you can elaborate. Compose a simple cover note that an editor can scan quickly. Four sentences will do. Press release enclosed. This format may sound abrupt, but editors don't have time to read long cover letters.
Faxes have a certain urgency, and short ones with simple formats just beg to be read right away.
They wind up in the garbage anyway, so don't waste your printing and postage. Send additional material only upon request. If you address it to a specific person, chances are you'll be sending your press release in the wrong direction. Come up with one or two meaty lines and attribute them to yourself. This gives the copy a more human touch and a more objective feel, as though written by someone else.