Companies that hire you as their courier expect you to give up your luggage allowance and travel only with a single bag or carry-on.
The duty of a courier is to carry shipping documents for the company hiring them and in return, airfare is significantly cheaper.
These companies use couriers because it is quicker and less expensive to ship freight as luggage rather than air cargo. Couriers may also carry time-sensitive items such as business documents or medicines.
You can only bring carry on luggage as a courier. Your luggage allowance will be used by the courier company to ship materials overseas. That's why you had to give up your allowance.
Courier tickets are regular round-trip coach tickets that are also non-refundable and non-transferable. So make sure you can make the flight.
Most likely, you will not keep any frequent flier mileage earned. It depends on the policy of the courier company. On the bright side, the length of stay for courier tickets is usually a seven-day stopover.
Sometimes, but rarely, length of stays can last up to six months. At the end, you may need to be a courier on the return flight home. This will be difficult if you plan to do any shopping overseas.
Sorry, but you probably won't be going to an exotic island on your courier flights. That's because most of the items you will handle will go to the main business centers of the world.
It is possible to travel with a companion, but you need to be flexible with your scheduling. It's rare to have courier seats together on the same flight. However, you can time it so you could meet your companion within the same day.