Courier travel was common a couple of decades ago and previously. You’d get your travel costs offset or even fully paid for in return for giving the company your baggage allowance, essentially, and transmitting accompanying documents between company representatives at origin and destination airport. That’s pretty rare any longer, and even some years ago courier companies preferred their vetted “regulars”.
Not long ago, a new company,
Airmule (link), popped up. Iirc it had a very limited network, mostly Asia, and had packages delivered to one at the airport that one would turn over to a contact on the ground at the other end. The concern many had was that the courier had no idea what you were couriering (some Asian countries offer up to the death penalty for some drugs and weapons) and the company would not guarantee they’d stand behind you.
They don’t buy your ticket - they pay you to carry packages they delegate to you. Years ago I did a bit of courier flying for the US Military. Personally, I’d never take on the risk of signing on with AirMule. “I’m sorry, sir, but xxx was detected in your baggage, and it (is prohibited, is subject to duty); please come with us.” “Yes, we understand someone paid you to transport this as your own baggage. But it’s your baggage, listed on the baggage sticker attached to your ticket.” Etc.
Check
Airmule out - but I’d suggest caveat emptor.