Its not ‘FAA law’, it’s DOT regulations. And those DOT regulations don’t apply to travel agencies. And it doesn’t have to be ‘refundable’ per say. AA, for example, used to not allow a refund, but instead allow a free 24 hour hold before purchase - which is allowed under the regs.

As mentioned by WineCountry, OTAs are not required to refund within 24 hours. Many do (Orbitz, Expedia, etc.) but that is used as an incentive for people to book with them, but isn’t required.

The fare class availability doesn’t change the fare you get returned unless you are booking for a number that exceeds the availability. For example, if it shows Y9...W2 S1 T0, and there is both a valid W and S fare filed, if you’re booking for only yourself, you’ll get the S fare (unless the W fare is cheaper - then it will show that - it’s not that common, IME, to have a fare basis in a higher fare class with a lower fare, but it does happen sometimes). If you’re booking a party of 2, then it will only bring up the W (unless you book on separate PNRs - that’s the only way to get the last S fare and then the W for a lower price).

Interestingly enough, I had a first that I can remember when searching for fares to LAX yesterday.

I initially looked on the app in the morning, and saw a G outbound/L return for flights that were good for me. Both the outbound and the return were showing 9s for the relevant fare classes. But wanted to book on the full website since I was going to use some TravelBank funds. Returned in the early afternoon time frame, to see the fare increased from $297 to $360 in the few hours. But the most surprising thing is it was still a G/L combo. Every time I’ve seen the fare increase, it’s always been a fare class thing (ie, they sold out/zerod our inventory and so the L become an S, or whatever, or I went over the advance purchase deadline and so was booked in a different fare basis). Since I wasn’t paying attention to the specific fare basis, just class, I can’t be sure, but I’m pretty sure it must have changed - either they eliminated the fare basis that I was looking at in the morning, or they raised the fare on it (by quite a bit) - I suppose it’s also possible they eliminated allowing a combination of that L fare I saw earlier with a G between my searches, but that also seems unlikely. I know it was the return part of the fare that was the issue, since a lower fare still existed on a G/G return trip with a different return flight. I know min/max stay or advance purchase wasn’t the issue, as the searches were done the same calendar day. I’m not sure what caused the L fare basis to change, but it must have. I booked this morning on what became a G/K combo, at $20 less than the higher amount above, but I know we took up all the return k inventory now, and it’s back to the G/L on our flights at $360 now.