My US airline has an agreement with Delta. It sounds like a ZED fare, but it is not called that.
All of the prices do not include the national/airport taxes, fees, customs charges, etc.
$50 one way/$100 within the 50 states (works out to about $150 with taxes/fees).
$200 round trip to Canada/Mexico/Caribbean.
$450?+ to Europe....and more than that to Asia and Australia.
It is space available, so if the plane is full of paying passengers you cannot go.
The other thing we have in the US is "Jumpseating". Pilots/Cabin Crew/Dispatchers/FAA, etc can "Jumpseat". You fly in a seat, if available, for free...and most US carriers all do this for all the other airlines. If you are in "CASS", an Identification system, then pilots can ride in the actual cockpit jumpseats, if no PAX seat in the back is available....and many larger planes have two cockpit jumpseats. Cabin Crew can typically use the cabin crew jumpseats if no PAX seat is available.
cliff
KHSV