Sylvia Roberts was on a Vermont ski holiday a couple of years ago when a chance remark caught her attention. A friend was talking about an inexpensive way to get to Europe: as a courier.

Returning to Manhattan, where the native Londoner works for a British commodities brokerage firm, Roberts got a courier application. "I filled it out and sent it in, but really didn't expect to hear anything."

Three days later, a Thursday, she received a call, "and I was on my way to London $200, round trip the following Tuesday." That was early in 1983. Since then, Roberts has made the New York-London trip as a courier four more times, and flown to Zurich for a ski vacation in Austria.

New York police detective Kevin Sutton ("I have a little more hair than Kojak") also has flown as a courier to London. One memorable adventure: When someone else was assigned the same seat on his flight back from London, Sutton was allowed to travel in the Concorde cockpit. ("The pilot, the copilot, the flight engineer and me!") Most recently, Sutton traveled as a courier to Rio.

Roberts, 45, and Sutton, 37, are two of the more than 5,000 people lined up by courier coordinator Jane Moschitta each year to accompany TNT Skypak pouches on flights across the United States and around the world.

TNT Skypak, part of a more than $3 billion-a-year Australian transportation conglomerate, is one of a rapidly growing number of businesses offering specialized delivery of air cargo. It is one of a few firms that uses free-lance, on-board couriers.

Another courier firm, Stratus Transportation Services, which now specializes in flights between Newark and Los Angeles, and Los Angeles and Chicago, plans to initiate service to London.

Free-lance commercial couriers -- as opposed to full-time professionals such as the State Department's diplomatic couriers -- go through the same boarding procedures as other passengers, including the routine X-ray and metal-detector security checks. State's couriers always are the last on and first off the aircraft.

Free-lance couriers come from all walks of life -- doctors, lawyers, mailmen, teachers, brokers. They do, however, share a love of travel, a yen for a bargain and, in cases where they're called on short notice, an understanding employer and the ability to leave on short order. (TNT couriers may request travel on specific dates or register on an on-call basis. They are not penalized for turning down flights.)

The Harlem Globetrotters' Robert "Baby Face" Paige, 30, and his wife, Zina, flew as couriers on consecutive days last May, from New York to Frankfurt. "I had this idea of going to Stuttgart and bringing a Mercedes-Benz back."

The Paiges didn't buy the Mercedes -- too much red tape -- but they did some sightseeing for their week in Germany. No pictures, though -- "It rained the whole time."

The flight to Frankfurt was uneventful and comfortable, even though Paige, the tallest Globetrotter, stands 7 feet. "I got the exit aisle seat and there wasn't anything in front of me. Lots of leg room."

Why would someone who travels all over the globe, and who clearly can afford to ride full-fare, travel as a courier? For the novelty of it, says Paige, "and for establishing contacts as you go along. You have to give thought to the future. One of these days I might not have the money. Then the contacts I've made could be of help.

"If I establish myself as a good, reliable courier now, that might help me in the future."

Frank Sansone, 25, who works for Eurobrokers, Harlow Ltd., a commodities firm, has accompanied pouches to Frankfurt, Geneva, San Francisco and Zurich. "All the flights went beautifully," he says, "with one exception. As I was going through the security check at the Frankfurt airport, the guards sent my suitcase through the X-ray machine. They gave me a startled look and several police officers hurried over."

Sansone had bought a cuckoo clock on a trip through the Black Forest and packed the heavy metal weights that work the clock in the bottom of his bag. "Under the X-ray, they looked just like a pair of grenades. We all were relieved after the bag was unpacked and they saw what it was."

The experience didn't dampen Sansone's enthusiasm for courier travel: He's booked on an upcoming Rio run and is planning a trip to London this summer.

Moschitta, 24, has been handling TNT's courier coordination for just over a year. "London, Paris and Rio," she says, "are the most popular destinations." Least requested are Frankfurt and Amsterdam.

None of the firms advertises for couriers, and, in many cases, employes are given preference. Outsiders usually hear about the opportunity from friends in the companies or by word of mouth.

"The nature of this business," says Moschitta, "is 'No surprises.' Our flights have to be filled and we do it far in advance five to seven weeks ." Couriers do not pay for the actual tickets, she stresses, but "to assure reliability," they pay a nonrefundable "administrative" fee when they're accepted for a flight. When there were no fees, couriers might fail to show up. Now, she says, "they have an investment to protect as well as an obligation to fulfill."

Once accepted as couriers, the travel procedure is simple and, except for the occasional long wait, painless:

On the day of the flight, couriers report to the TNT gateway office, usually located near the airport, three hours' ahead of time. While there, they are given their ticket and, shortly before flight time, they and the pouches they are accompanying are taken to the air terminal and checked in.

The pouches -- anywhere from 15 to 75 of them and weighing 70 to 100 pounds each -- are sent as the courier's personal (and overweight) baggage. Pouches could contain business documents, computer chips or even Cabbage Patch dolls. The courier, who doesn't know what the pouches contain, is responsible only for the accompanying documentation.

On arriving at their destination, the courier and pouches are cleared through customs -- generally, says Moschitta, within half an hour after arrival.

Prospective couriers must be at least 18 (most are in the 18-50 range), in good health, properly attired -- "business suit or sport jacket and slacks for men; dress or suit (pant or skirt) for women" -- and properly documented -- "valid passport and visa, where applicable."

TNT couriers are limited to one piece of carry-on luggage ("It's taught me to economize when I pack," says Roberts), prohibited from purchasing duty-free goods other than liquor and are not allowed to consume alcohol during the flight.

"In all other ways," says Moschitta, "they not only resemble regular passengers but are treated as such."

Roberts, who has been with her firm -- Woodhouse, Drake & Carey Inc. -- for 23 years (the last six in New York), likes to point out that her five round-trip flights to London ($200 each) have cost the equivalent of one full-fare round trip, and less than the full one-way fare via Concorde, on which she has traveled five times as a courier ($225, round trip).

"I was a little apprehensive before my first trip as a courier," she admits, "but it was so much easier than I thought."