CONSUMERS : It Can Pay to Carry Extra Baggage : Travel: Budget-minded make worldwide deliveries for courier and transport companies to help cut vacation costs.

Looking for ways to save money on that vacation? Serve as a courier and go overseas for a pittance. Or hire on with a car transport agency and drive cross-country for next-to-nothing.

More and more budget-minded consumers are discovering that the proliferating overnight-mail market can offer them low, sometimes downright cheap, air fares to many international cities--and a few domestic spots--if they will serve as a courier for cargo or document shipments. For example, how about $300 for a round-trip fare to Singapore that normally costs $1,400?

Stateside, if you prefer to drive, auto transport companies may be the ticket, possibly even for a trip in a luxury vehicle like a Jaguar or a Mercedes. For deposits ranging from $100 to $250, depending on the agency, you can deliver someone else’s car to most major cities nationwide and see the sights. You’ll drive mostly newer models. The transport firm gives you the first tank of gas free, though you’ll pay the expenses thereafter.

What’s your price for this inexpensive travel? You will have to bend your vacation timetable to match that of the courier or car-transport firms. You may be required to leave on short notice or alter your schedule if the company suddenly postpones your trip. (You’ll usually be rescheduled to travel in that same week.)

“The No. 1 thing passengers interested in being couriers should ask themselves is, ‘How flexible are they?’ ” said Dan Rosendahl, owner of Way-To-Go! travel services in Los Angeles, a firm specializing in booking courier flights, air charters and standby seats. “The second thing they should ask is, ‘Is the (fare) savings worth the restrictions?’ ”

Prospective couriers also should ask about fine print, Rosendahl advised, explaining: “There are general types of restrictions. No pre-seating, no special meals, or transferable tickets.”

Valid passports are a must, of course; most firms require that couriers be at least 21 years old and have a clean and conservative appearance; some firms bar them from drinking alcohol before or during flights.

Couriers, generally, must travel solo, although some firms will help schedule a traveling companion on another flight the same or next day. Couriers get their tickets by meeting an agent, usually at the airport a few hours before their flights depart.

Courier fares vary by season, reflecting regular ticket costs; the savings are greatest during off-season.

Being a free-lance or “casual” courier usually doesn’t mean much hard work. Most couriers don’t even have to lug a package in exchange for their fares, usually half- or a third-off regular ticket prices.

Still, there are exceptions, said Sherri Dahl, of IBC Pacific Inc., a wholesaler of courier seats. In Taipei and Seoul, she said, customs agents require couriers to personally escort baggage through.

Be sure to ask if a firm charges a deposit for your trip. IBC, for example, requires “casual couriers” to deposit $500 before booking a flight; the money is returned when the passenger returns. Other courier firms charge $100 to $300 in deposits, which are returned at the destination or on stateside return.

Most of the time you’ll have to lug your own bags on the plane since the courier company is using your baggage space for its shipment. This ensures that their cargo is loaded immediately and is processed through customs faster than it would be if shipped by air freight.

“Some airlines will allow the courier to check one or two bags” besides the firm’s shipment, said Jo Mabey of Excalibur International Courier Inc., which contracts with Way-To-Go! for couriers. “Or (couriers) can always pay extra for it.”

On Friday, Sachit Shah of Los Angeles, discovered another benefit to courier work on his initial travel this way. He met Mabey at TWA’s counter and received his ticket to London, bypassing the regular, international passengers waiting to check in. In minutes, he was handed instructions to go to the London airport courier office, then was dispatched to the passenger waiting lounge.

Why did he seek courier work? “I found an ad for this in the newspaper, so I thought I’d try it,” he explained, adding he “saved around $150” on the regular round-trip fare to London, which at present, tourist-season peak is about $840.

Prospective couriers should know there are other wrinkles to the work: They may accompany shipments one-way or round-trip; they may go for one or two weeks, 30 days or on an open-ended ticket.

Most firms book two months ahead and currently have little or nothing left for July for popular spots like London. But they do have cancellations, so if you are ready to fly at the last minute, keep calling.

“We sign up people who can go at the last minute, and they’ll get a really low-cost fare or free,” said Rosendahl. “We just had a recent last-minute to Singapore for $150 round-trip.”

He does not charge registration fees for booking low-cost flights. But he has set up a separate travel club for $75 a year; members can get $50 to $100 more off on their flights, he said, adding that charge is worth it only “If you travel more than once a year. . . . “

Some agents charge to book courier flights. Now Voyager, a large New York agency, for example, charges a $50 membership fee.

The motor-minded--those interested in driving a car, van or truck across country--should know that most firms will not take reservations more than one week in advance of departure.

From Los Angeles, drive-away agencies report, the most popular destinations are New York and Florida; those bookings should be relatively easy. But drivers beware: Most drive-away firms will not take personal checks for the $100-to-$250 deposit they may require.

Depending on company rules, drivers get specified time spans to get to a destination. Auto Driveaway Co. in Los Angeles gives drivers eight days to New York and Miami; nine days to Boston; four days to Seattle.

“The time is based on driving about 400 miles a day, about eight hours each day,” said Jean Moen of the Wilshire Boulevard firm, which has been in business since 1952 and has 90 offices in the United States and Canada.

Moen said her firm requires drivers, who are fingerprinted and photographed, to: hold valid licenses; be 21 years old, although some other agencies will hire 19-year-olds; and provide references here and at the destination.

“A lot of people do this,” Moen said. “They like driving one way, then flying back. And we get a lot of international travelers now, too. It’s a reasonable way for them to get across the country and to see something of it.”

She said her firm urges owners of older-model vehicles to ship them by truck, meaning drive-away motorists mostly deal with newer, nicer cars. “But we get all kinds,” she said, adding, “We had a Lamborghini the other day.”