Dear Do-It Man: I received in the mail an award certificate from a company in Houston, Tex., called National Clearing House. The certificate informed me of the company's nationwide promotion in which I was a confirmed winner of at least one of these prizes: a 1993 Dodge Shadow; a 10-carat sapphire & diamond tennis bracelet; a 46" wide-screen color T.V.; five-day and four-night Hawaiian accommodations; $2,500 in cash.

I must call the company within 72 hours to validate my claim.I operate a small business. I am the author of a dictionary based on phonics called "Tha Amaerkan Dixunaery."

R.W., Magna.

Dear R.W.: National Clearing House sells advertising materials to businesses. We expect you're on the company's mailing list because of your dictionary business.

The company says this promotion is a way of introducing potential customers to National Clearing House and its products. It sells 50 different products a business could use to promote itself.

Here's how it works:

A customer who calls the company (the call is at the customer's expense) in response to the promotion will be asked to listen to a sales presentation. Examples of products National Clearing House sells include executive ballpoint pens with up to five lines of printing, $3.99 apiece; and an engraved Cross or Parker pen and pencil set with a display case, $65.

The company says you don't have to purchase a product to win a prize.

A National Clearing House spokesman told us on the phone he couldn't attach a probability to the chance of winning any one prize because that will depend on the number of people who respond to the promotion. But the Better Business Bureau in Houston told us most companies will receive the least valuable prize. (The BBB also advises people to shop around, comparing prices, before making a purchase.)

The National Clearing House spokesman told us the company had already awarded diamond and sapphire tennis bracelets and "quite a few" Hawaiian accommodations. He said the company would award the car, the TV and the $2,500 in cash in a couple of weeks, after the company's first fiscal year ends. The company started business last March.

The National Clearing House spokesman estimated the value of the Hawaiian accommodations, which he said are deluxe, to be between $500 and $750. He would not tell us the value of the diamond and "midnight blue" sapphire tennis bracelet. "It's against the law to give the price of jewelry over the phone," he said.

A local jeweler laughed when we asked him about "the law." "We've been in the jewelry business for 35 years and I've never heard of such a law. I can't imagine a law like that," said Larry Limb of Limb Jewelers Inc.