Among firms using free-lance couriers:

* Stratus Transportation Services, 858 Burlway Rd., Burlingame, Calif. Minimum age, 18. Reservations desk, (415) 340-1793, 12:30-5 p.m., Washington time. All night flights, Monday-Friday. Nonrefundable administrative fees: Oakland-Newark, $160 round trip, $80 one-way; Oakland-Los Angeles, $50 RT, $25 one-way; Oakland-Chicago, $160 RT, $80 one-way. Administrative fee must be paid within five days after reservations (no more than one month in advance) are made. Couriers sent contract and receipt by mail. Reservations not changeable.

* TNT Skypack, 146-92 Guy Brewer Blvd., Jamaica, N.Y. 11434. Minimum age, 18. Direct inquiries to Jane Moschitta, (718) 656-3212, 9:30-11 a.m. Flights include New York to: Amsterdam, Frankfurt, Geneva, Paris, Rio de Janeiro, London, Houston, San Francisco, Los Angeles; San Francisco to: Hong Kong, Sydney; Chicago to: London, Mexico. Nonrefundable administrative fees for round trips: $50 (JFK-Houston) to $255 (San Francisco-Sydney). Reservations generally made 30-45 days ahead of time. A strict turn-around time of one week required for the New York flights; a 10-day stay required for the San Francisco-Hong Kong and Sydney flights. Reservations not changeable. Travelers limited to one piece of carry-on luggage (total dimension of 45 inches.)

For other firms that might use free-lancer couriers, look under the Air Courier Service or Air Cargo & Package Express listings in the Yellow Pages. Also, check with the Better Business Bureau before paying advance "membership" or "administrative fees." While most courier organizations are legitimate, some have charged advance fees and then gone out of business.