For cheap fares without the compromises required for freelance couriers, check out ticket consolidators.
Sometimes billed as "discount travel agents," consolidators offer the travel equivalent of factory outlet stores.
These companies buy last-minute empty seats on flights at steep discounts, which they pass along to consumers who know enough to ask.
They also buy bulk discount tickets for tour groups and package them with hotel or rental car deals. If the tour groups don't fill up, the remaining tickets can land in your travel plans at lower prices than the official airline rates.
The downside is limited travel times and dates. And unlike with regular tickets, airlines face no obligation to get you to your destination if bad weather or mechanical problems mean delayed or canceled flights.
This can be particularly nettlesome on return flights. To get home, passengers may have to purchase an "upgrade" - pay more for another ticket on a different flight. And this new ticket will not likely meet airlines' 21-day advance-purchase discount requirements.
Some airline consolidators:
American Travel Consolidators
Cheap Fare$ Express