is there a thrifty shuttle bus from seatac international terminal to your car rental facility?

One bus for all the companies, they're all in one building

Yes. But sometimes the shuttle is very crowded.

Yes. There is a shuttle that takes you from the terminal to the off site rental facility. You can pick that up outside the doors from where the baggage claim is. Look for the signs to direct you where you can board the shuttles. Its a little confusing since, at the time of this writing, there's some construction taking place. I, like you, was just a customer. I don't work at or own the facility. Just a visitor from Orlando.