Please can you explain the US connotation of "standby". It has featured in several other posts. Here in the UK standby was an old ticketing method of waiting at the airport without a reservation, and if there were seats still available when everyone with reservations had checked in, which there usually were outside peak times, you got them at a cheaper than normal fare. BA used to have "standby desks" at Heathrow, with students etc hanging around. It's all gone now.

What you have in the US, with fees being sometimes charged, sems to be something different under the same name

Sorry for such a simplistic question.