It used to be very common, but improved routes from FedEx/DHL etc and increased security have ended the business.

https://www.gonomad.com/6364-air-courier-flights-have-lost-t...

http://courierlist.com/ -- which has an announcement from 2005 saying the discounts aren't worth it any more!

When I flew to Amsterdam in 95, guy next to me had a career as a courier for Ford and GM (I flew from Detroit). He mostly brought over specialized tooling parts for engineering depts in US and Europe. Always a carry on he says to prevent theft. He was in his 60's back then but said he's been doing this for a number of years and got to travel the world. I guess these companies still do it.

It happens in the space industry too. I've never seen a dedicated career courier, but rather someone who worked on the project handles it. It's useful when the project is late and the person csn head out for the flight immediately without any shipping company time overhead.

Back in the 90s I knew people who would fly back to Japan carrying parcels, in return for reduced tickets (not $99 though, but it might have been £300 instead of £400). I always thought it was kind of sketchy, but as I understand it what was carried were documents, and no one I know ever got into trouble.