Was feeling lazy yesterday flying SEA-SFO with a cat in F on a 757-300. The website generally won't allow an in-cabin pet due to limited underseat storage, but I was in 1A with the massive cutout.

I checked-in online, went through security and showed up at the gate with my cat. Paid the $125 there, got a neon tag, and was able to keep my first class seat. Easy stuff.