Message is the Medium for Discount Air Fares : Couriers: In return for accompanying a package or documents, you pay 50% or more below ticket prices.

Last July, about the time I was spending around $1,000 to fly to London and back, my friend Ana- lisa was winging her way south on a $350 round-trip ticket between Los Angeles and Sydney, Australia. About three months later, while I was spending $555 on a Los Angeles-New York round-trip ticket, my friend Michael was following in Analisa’s Australia-bound airstream at the same $350 price.

The secret of their cheap fares: They were acting as couriers.

Among the most seasoned and flexible travelers, signing on with a courier service has long been known as an entree to travel bargains, with fares that often run 50% or more below normal ticket prices. You travel alone, pack light and fit your itinerary into someone else’s corporate schedule. In return for accompanying a package or documents--usually on both portions of a round-trip flight--you get a good price.

In these days of universal faxing and information highways, the courier’s trade might seem a candidate for obsolescence. But instead, business is booming. Byron Lutz, who tracks the industry for the International Assn. of Air Travel Couriers, estimates that 25,000-35,000 flights yearly leave the United States with a courier aboard. That figure, he and others say, is rising--because international commerce continues to increase, and because it takes more time and money to send materials unaccompanied than to match them with a passenger.

The result is an opportunity for international air travelers who are willing to accompany time-sensitive original documents and other entirely legal materials. And though New York remains the capital of U.S. courier travel, West Coast firms send couriers to a dozen major cities around the world, particularly along the Pacific Rim.

“Let’s say you were going to Bangkok,” says Toni Carpenter, international director for the Los Angeles-based courier firm IBC-Pacific. “You would sign an agreement with us saying that you would go on such-and-such date and return such-and-such date. You put up a security deposit to insure that you’ll come back when you’re supposed to come back. And then you report to the airport.”

A traveler usually needs to be 21 years of age and have a passport. Depending on the country of destination, a visa may also be required.

Since courier companies only need one seat per flight, the traveler usually must be willing to go alone or have companions pay full fares. Seats can usually be reserved up to three months in advance.

The traveler must pay the courier company well in advance, usually with cash, a cashier’s check or a money order, to hold his or her seat. (Courier companies, which have contracts with airlines that allow them to buy tickets at deep discounts, usually buy seats in travelers’ names, and hold tickets until handing them over at the airport on the day of departure.) Often the courier firm asks the traveler to also put up a refundable deposit of $100-$500, which might be a personal check or a credit card number. (Would-be couriers who cancel forfeit a portion of their fare, the amount growing larger as departure date draws nearer. Couriers who fail to arrive on the proper return flight lose their deposits.)

Since the courier service is using the traveler’s in-the-hold baggage allotment, the traveler usually must be ready to fit all he or she needs into a single carry-on bag, small enough to fit in an overhead bin or under a seat.

Travelers are usually asked to appear at the airport two or more hours before their departure time. Only then do they receive their ticket from a courier company representative. Usually, the traveler receives only tickets and a manifest, and never sees the baggage that is nominally his or hers.

Many first-timers are wary about what exactly they’ll be accompanying. The cargo is usually paperwork--business papers, legal papers, blueprints and so on--and occasionally small objects such as computer parts. The materials usually have little value except to the sending and receiving parties.

U.S. Customs officials in New York and Los Angeles say they’ve had no problems with contraband associated with courier companies. Still, Customs spokesman Mike Fleming in Los Angeles urges that travelers “make sure that they check out the company, and that everything’s kosher.” One way to do that is to ask how long the company has been doing business and what airlines its contracts are with. For still more reassurance, offer to make your advance payment in person at the courier company’s offices. They may appreciate the chance to have an advance look at you, too.

After clearing immigration overseas, couriers usually hand off their folders to a courier company representative in the customs area. (Many companies give travelers pouches with logos on them, so that couriers can be recognized when they arrive.) Some couriers never even see a baggage-claim room; others spend a few aggravating hours waiting for their cargo to clear. Once through customs, travelers are on their own until their return flight.

Often, travelers are asked to pledge that they won’t drink any alcohol on their flight. (Courier companies don’t like the idea of staggering drunks carrying their logo around.) Shorts and suggestive T-shirts are often forbidden, and sometimes so are jeans or sneakers. Backpacks are often discouraged because they are thought to attract extra attention from customs inspectors. When an inspection lasts longer, precious time is lost.

“Our livelihood depends on maintaining a good working relationship with customs,” says Carpenter. Hence, over the last decade, she and her IBC-Pacific colleagues have compiled a list of about 25 former couriers with whom, for one reason or another, they won’t do business again.

Anyone serious about flying as a courier should do further homework. Kelly Monaghan lays out the details of the air courier trade--and lists courier companies and their offerings--in his 218-page book, “The Insiders Guide to Courier Travel: How to Travel World-wide for Next to Nothing” ($14.95; published by The Intrepid Traveler, P.O. Box 438, New York 10034). Even though the book has been recently updated, readers should note that courier company destinations, schedules and prices change frequently. Beyond the overview he provides of the business, the most valuable information in Monaghan’s book is probably the list of addresses and phone numbers, which allows would-be travelers to call companies for up-to-date information.

Another source is “Airfare Secrets Uncovered: The Insiders Guide to Huge Discounts on Air Travel,” by Sharon Tyler and Matthew Wunder ($14.95, published by the Universal Information Corp., distributed by ProStar Publications, P.O. Box 67571, Los Angeles 90067; 310-287-2833). Tyler and Wunder’s book, spiral-bound and oversized, is broader in scope, spends 37 of its 150 pages on courier travel, and elsewhere discusses airline ticket consolidators, frequent flier programs and other discount possibilities.

A third source is the International Assn. of Air Travel Couriers (P.O. Box 1349, Lake Worth, Fla. 33460; 407-582-8320). For a $35 annual fee, the association sends members “The Shoestring Traveler,” a six-times-a-year newsletter that includes updates on courier offerings and other discount travel opportunities.

What follows is an unscientific sampling of West Coast sources for courier tickets. Among the lowest round-trip fares from Los Angeles available when I called around last week: to Sydney, $450; to Hong Kong, $450; to Tokyo, $299; to Mexico City, $175; from San Francisco: to London, $450; to Bangkok, $475. Some of these companies also handle departures from Miami and New York. Most companies are open during weekday business hours and change their prices often, so consider the fare figures above only a general guide.

* SOS International Courier (8715 La Tijera Blvd., Los Angeles 90045; 310-649-6640).

* Way To Go Travel (6679 Sunset Blvd., Los Angeles 90028; 213- 466-1126 or 619-224-2277 or 415- 864-1995). A booking agency that arranges flights with three courier companies, using United, Singapore and LACSA airlines. * POLO Express (811 Grandview Drive, South San Francisco 94080; 310-410-6822 or 415-742-9613). Works with United Airlines, allows no upgrades or frequent flier miles. * IBC-Pacific (1595 E. El Segundo Blvd., El Segundo 90245; 310- 607-0125. Closed on weekends and Mondays). Uses Northwest, Singapore and Continental airlines. * Midnite Express (925 W. Hyde Park Blvd., Inglewood 90302; 310- 672-1100. Call between noon and 5 p.m. on Tuesdays and Thursdays). Citing troubles with late-arriving passengers from other areas, the company says it will only use Los Angeles County residents as its couriers.