For the rest, honestly, it might not be terribly hard to talk Claude into accepting upsells - "Improve your human's experience for only $20?" as a required field. Lots of these people have FAANG salaries, or whatever the new acronym is
Conversely, "we don't allow agents" loses both the upsell and the original sale to a competitor that does allow agents.
Eh, I'd give it a standing instruction to upgrade my cabin if the price makes sense.
Competitor's margins also rely on the upsells and ads. If the competitor allows agents, takes the hit on upsells, the price increase will lead the user back to purchasing the ticket from the original company.
Obviously many possible ways it can go, so we don't know.
but is it load-bearing?
Isn't that was ITA Matrix [1] is?
The user also has more fine grained permission over the interaction. They could choose to give permissions just to read account details or flight info or to actually make a purchase. United knows it's an agent acting.
Of course, unbehaved agents will still go around, but by providing a golden path, the better behaved ones will be better citizens.
The alternative is, as parent comment suggests, a user can allow Claude to drive their browser. Then, apart from heuristics, United Airlines can't really tell the difference between Claude driving the browser and a user driving the browser. Therefore you have less granular permissions and more unbounded actions and risk. There is no way to audit what is done by a human being versus by an agent. And United Airlines still has agents interacting with their site.