Many years ago when I first started travelling, one of the tips you heard for a great way to get cheap international air tickets was to travel as a courier carrying items for specialized courier companies.
I don't think these were companies like DHL or FedEx but I think they were proper companies complying with all relevant regulations and were not regarded as something sketchy or dodgy.
The idea was that you carried personally on the plane something fragile or important to be delivered in the other country. This counted as part of your on-board luggage so you couldn't bring anything or could bring less on board of your own stuff. I'm not sure if checked luggage was also ever used. In return you got a free ticket or a ticket at greatly reduced cost.
But now I haven't heard of this travel option for years. Did it disappear due to changing rules after international terrorism? Or does it still exist? Does it only exist in some places?