American Airlines offers a $41 early-boarding fee.

But, you already have a seat assignment. Maybe you can ensure that you get to stow your carry-on forward of your seat (instead of behind you) - but many of AA's planes have upgraded overhead compartments that allow vertical stowage of roll-aboards, which is more than adequate for carry-on luggage on a full flight.

So, really, one is paying American Airlines for the privilege of standing in-line on an unconditioned jetway instead of a somewhat-conditioned terminal.

Caveat emptor.