Well, first thing to notice is that $426 + $30 = $456 vs $536 which is only $80. The more you chip away at it (maybe with Avis discount codes of some kind if you haven't tried that already), the less that difference may get. I actually have zero experience with Advantage, but it doesn't sound good and would probably be inclined to spend the $80 if I had exhausted other options for getting a better price from Hertz, Avis or National. Heck, I'd probably try others like Budget and Dollar first. I've had reasonable experiences with Dollar in the past (though I haven't used them recently).

Anyway, I gather you've tried different locations in Seattle? Given that it's a one-way, I assume you don't need to pick it up at the airport. With one-ways it might be trickier to assume which location is cheaper, but it's worth trying different ones to see if there's a difference in rates. And if you haven't already, have a look at the Avis forum for discount info.