A WEEK ago, Gulliver took a trip to California. His credit card allowed him to check in a bag free, but southern California doesn’t require much bulky clothing in late August so he made do with a carry-on. Still, in this era of luggage fees, it felt almost wasteful to let a baggage allotment go unfilled. It is probably no surprise to hear that there’s an app for that. And there’s money in it, too. Grabr, a startup in San Francisco, just raised $3.5m, some of which came from a Russian government fund, and has 50,000 users. If you find yourself with extra luggage space Grabr searches to see if anyone in the city you are flying to wants goods that can be bought in the city you are flying from. It then enables the transaction in exchange for a delivery fee. Requests on Grabr are diverse in their geography, type and remuneration. For bringing a Citizen watch to Vu in Ho Chi Minh City, you can make $5. Franz in Munich is apparently unsatisfied with the quality of coffee there, and will pay you $15 to bring him a box of San Francisco’s Blue Bottle beans. Ahmed in Cairo has seemingly spotted a good deal for an Apple Macbook Pro in Dubai, and is willing to part with $100 to have someone bring one to him. James in Bangkok has a hankering for Taco Bell tacos, but is offering just $5. (And frankly, James, I might caution you against eating tacos that have travelled around the globe.)

Airmule, a rival, sets standard fees for deliveries: $40 for packages up to 5lb (2.3kg), or $60 if the “mule” departs within 48 hours, plus a $5 fee if the mule has to travel to pick up the package.

There is logic to this evolving system. There are also obvious pitfalls, most of them legal. It is not against US government policy to travel with someone else’s possessions, but if those possessions turn out to be illegal, the carrier can be subject to penalties. Grabr largely skirts this conundrum by having the courier do the purchasing and packing. With Airmule, the goods come pre-packaged by the person sending them. One of the founders of Airmule told Lonely Planet that “restricted items are not allowed,” which may not fully reassure couriers. But senders also create a paper trail by uploading photos of the item before packing it and supplying their credit-card information and address.

Then there’s the issue of individual countries’ restrictions on imported goods. As Forbes notes, a courier bringing Serrano ham from Spain to the United States would face problems because of the restrictions on imports of cured meats. (Gulliver’s girlfriend learned this the hard way when she had a bounty of Barcelona ham confiscated on her return to Washington, DC.)

Apps already let you stay in someone’s home when it’s not being lived in (Airbnb), or rent someone’s car when it’s not being used (Getaround), or rent someone’s clothes when they’re not being worn (StyleLend), or even borrow a lonely dog (believe it or not, there are multiple apps for that). There are still a few kinks to work out, but it makes sense to create a way to rent excess luggage space, particularly when it has become such a valuable commodity.