If you can relay your sales pitch in 70 words or less, the least expensive way to get your message across is with a postcard mailing. Postcards are inexpensive to print, and mail first class for about a dime less than regular first class mail (bulk rate postage is the same for both).
But postcards have inherent limitations. Because there's no "pouch" envelope included, you can't really request and expect to receive many payments with your orders. You could use a double postcard to request people to respond by mail with an order and you'll bill them later. Generally, though, postcards work better when your objective is to gain inquiries rather than sales.
Recommendation: Use some form of graphic design to relay your message quickly. While you're hoping people will read the message, you can assume a high percentage of people will only glance at the first few words to make their toss or call decision. The graphic can be anything from a picture of your product to a dollar sign split in half to show how your prices are less than everyone else's. The more the graphic relates to how the product or service will benefit your prospect, the better your response is going to be.
Don't forget: Make it easy for your prospects to respond. Include your 800 number for a national mailing, your local number for a local mailing, and a fax line if you think people will fax requests in. You can even put a short message to fax your name and address for quicker response. Anything you do to make it easier for prospects to respond will help your sales effort.