Well, that's the way it always was, but increasingly being an air courier means paying, albeit a very small amount, for your ticket, which the air courier company provides in exchange for you accompanying documents and other valuable goods on commercial flights. Becoming an air courier means no or very low cost travel, but you can't check in baggage, as the courier company needs the space for the goods you are accompanying. You will also have to pay for your own accommodation the other end. Above all, you won't be able to plan your trip very far in advance.
The problem for the courier companies is that they get pretty short notice from their customers. In practise, that means maintaining a register of 'willing and able' couriers, so your best bet is to register with several air courier businesses.
You'll find their details and plenty of free advice on the Internet. Whilst not everyone is in a position to take advantage of this opportunity, it's a great way to go if you have the time and inclination.