They are supposed to charge you at check-in, but that's assuming you actually pre-reserved. The line-checkers in security (not TSA, but the contractors who monitor the line) are supposed to check for the pet receipt before letting you through. If you don't reserve the pet and manage to get through security without them noticing, it's very possible you could just get the pet on the plane without paying, since the GA almost never asks (since pets are supposed to be pre-paid at the pre-security counter).

I don't think there's an official weight limit but I'd say no more than 15 lbs. They have to fit comfortably in a small carrier.

It's wonderful that people have dissenting opinions, but can we please not turn this into another "I don't want pets on board" thread? This is about pet fees, and pet fees only; let's not rehash the age-old argument about whether pets should or should not be on board in the first place.

(FWIW, I travel with my dogs in-cabin once a year, and every single year, nobody on the plane even notices the dogs until we deplane. Even our rowmates are completely surprised when we pull the dogs out from under the seat. Never a problem with noise, and never a problem with allergies... if the pet remains in the carrier, there is very little danger of allergies. Noise is the primary issue and it can be just the same with children. But, that's the last I will say of this in this thread, because I really don't want this to turn into yet another pets versus anti-pets thread. It gets old and ugly really quickly.)

Speaking of pet fees, the newly-raised $175 fee means that next Xmas, we'll be looking very hard at traveling on other airlines where the fees may (hopefully) still be somewhat more reasonable. We may end up having to board the pets, but if AA keeps their pet fee at $100, then AA will be a very likely candidate for our Xmas travel next year, if the all-in price is sufficiently competitive.