One company, Airmule, pays $150 to travelers with a visa for China who are willing to “sell” a luggage slot on their flight there from the United States, or on a flight to India, Thailand or South Korea that stops in China. (Flights back to the United States pay $100.)

There are downsides, however: Couriers need to be at the airport up to three hours ahead of their flights, and they have to pay any baggage fees for the Airmule luggage, as well as their own. They also need to pay any duty that might be charged on the items they are carrying in advance. (The company advertises it will reimburse any duty as long as the courier submits the paperwork.)

It’s unclear what sort of support the company offers. Several calls weeks apart to the customer service number during advertised business hours only reached voice mail. Sean Yang, the founder, responded to an email requesting comment, saying he was traveling and didn’t have time for an interview.

Still, a few bloggers who have tried Airmule have given it generally good reviews. Though one did note that a certain amount of trust is involved. Another well-established air courier, Chapman Freeborn OBC (On Board Courier), addresses many of the Airmule shortfalls, but has its own limitations.

OBC books and pays for an economy plane ticket, as well as any luggage fees. It also pays a daily sum to the courier, and it has customs brokers to help at the destination, though the process can still take many hours, said Mr. Nikolai Bergmann, director of the OBC program.

It’s not for the casual traveler, however. The company chooses the destination and the flights, and usually gives couriers only a few hours’ notice of a job. And it’s generally a quick out and back, even on international flights.

“You really need to be an aviation nerd. You need to love airports and flying in order to enjoy this kind of work,” Mr. Bergmann said.