The gap after a booking confirms is the same shape everywhere: the traveller has committed, the seller has stopped selling, and somebody else collects whatever gets bought next. What differs is who that somebody is, and what it costs you.
In short
Post-booking ancillary selling applies to any travel seller, but the reason to do it changes. An online travel agency is recovering a sale that leaves for the airline's own site. An agency is turning a servicing cost into revenue. An airline is extending merchandising into channels its own site never reaches.
Five starting points
Online travel agencies have the sharpest version of the problem. Air margin is close to zero, the traffic is already paid for, and every extra bought after booking is bought somewhere else using the reference the OTA handed over.
Travel agencies and TMCs face the inverse. The extras are not lost revenue so much as unpaid work: an agent picks up the phone to add a bag, and that call costs money and earns nothing.
Airlines already merchandise well in the direct channel. The gap is everywhere else, plus the post-booking window even in direct, which is where travellers actually go to change something.
Tour operators sell a package rather than a flight, which gives them a much longer window and many more natural moments to offer something.
Rail and cruise operators have the identical gap and almost no vendor writing for them. The unit an extra attaches to is the itinerary, not the flight, which is exactly why several categories travel across modes unchanged.
Frequently asked questions
Why separate pages rather than one?
Because the problem is genuinely different. An OTA is losing a second sale it never made. An agency is carrying a servicing cost with no revenue attached. An airline already merchandises well in one channel and not in the others. The same cart answers all three, but the argument for it is not the same.
Which page applies if we are more than one of these?
Most sellers are. Read the one that matches where the pressure is now rather than the one that matches your company description.
Is this only for air travel?
No, and the rail and cruise page is the honest version of that answer. Some products are air specific and some attach to an itinerary regardless of how the traveller is moving.