Just because it's being done by different companies it doesn't make it right. It's a shady way to lure customers and mislead them. They manipulate these fees in order to have a lower price displayed on the search results.

They are disclosing them, HT isn't including them in the prices they're showing the users though. C.f. the video I posted. What you see before I start scrolling is the entire screen that's visible to the user, with the Apple Pay button at the very bottom.

The fee is disclosed in the small print on HT. But it's not part of the total you pay to HT.