score:2
Depending on the day and how busy they are, you might be able to just check them in for your next trip. I did it quite early for one from Seattle to Portland as well - very similar trip actually. As you approach the ticket windows, on your left is the baggage check in and the guy will give you a ticket for it.
Alternatively, if they say it's too early, they do offer a checked baggage storage option (click the 'baggage' tab half way down the page).