American Express testing a service for speedy letters

American Express is returning to its roots. The giant conglomerate, perhaps best known for its green-and-gold travel cards, is testing a novel express-mail service here. If it is successful, the company intends a nationwide expansion of the delivery system, which guarantees one- or two-day delivery or double the postage back.

When American Express started in the mid-1800s, it was a freight company that delivered mail in the Eastern United States until the US Post Office was formed. It even had its own stamps. Two of the partners who founded the company also started a cartage operation in the west called Wells Fargo and ran the fabled Pony Express.

But last year, when a company called Western Airletter approached American Express with a proposal that the two companies get back into the mail business, ''frankly we were skeptical,'' says Akin Toffey, vice-president of American Express' new products division in Denver. After studying the situation further, however, the company decided to give the idea a whirl.

''Airletter Mail Express: The New Way to Mail'' is a joint operation by American Express, Western Airletter, and Burlington Northern Air Express. It differs substantially from Federal Express and the other guaranteed-delivery operations that have sprung up in the past several years. It is aimed at consumers and small businesses, rather than large companies. It works with the US Postal Service rather than bypassing it entirely. Also, it takes only letters , no packages.

Customers will take the letter, check, or application which they want delivered promptly in another part of the country to a participating retailer. In the Denver test, grocery stores, photo-finishing outlets, and savings-and-loans are offering this service. There the person buys a large stamp - $6.50 for one-day and $4.25 for two-day delivery - puts it on the letter, and leaves the letter at the store.

At the close of the day a truck picks up all the letters and takes them to the airport. There, the Airletter people sort them, check the postage, and get them loaded on the right airplane. At the destination, the letters are delivered in ordinary fashion by the US Postal Service.

''A letter from Los Angeles to New York can take anywhere from two to seven days days by ordinary mail, because it can take anywhere from one to six days to get to the airplane,'' Mr. Toffey explains.

The Postal Service moves 110 billion pieces of mail yearly. That's a pile of mail as big as Pike's Peak and more than a person can count in a year nonstop. Each piece of mail goes through 37 steps between collection and delivery, and 34 of these steps take place before the letter reaches the airplane.

''It's a phenomenal job and the post office is amazingly efficient,'' the American Express manager says. ''A 20-cent stamp is one of the best bargains in the marketplace.'' The Airletter operation uses the most efficient portion of the US mail system, he explains. Barring highly unusual circumstances, like major blizzards or airplane crashes, Airletter feels it can deliver every letter within the 24- or 48-hour limit. But if it can't, it will give you a replacement stamp and refund the money you paid - in effect, doubling your refund.

According to Mr. Toffey, the marketing indicates two types of customers: individual professionals like doctors and lawyers, and consumers at home who have payments or applications that must meet a deadline or who want to make sure a letter or card arrives in time for a special occasion like Mother's Day, a birthday, or Christmas.

The test began Jan. 10 and will run for four months, after which a decision will be made on whether to expand the service nationwide.