Just be sure you have the Emerald agent print you a copy of your rental agreement...you'll need it for the garage security guard at SEA. Otherwise they'll either send you back upstairs or have to make a lengthy phone call with the National desk.

SEA is the only place I've seen where you need a printed contract to exit the garage; elsewhere, I've just had to show my driver's license and hangtag to the exit booth agent (half the time, they don't even ask for my credit card) and am on my way.