American Airlines has round-trip flights from Dallas (DFW) to Seattle (SEA) for $178, NONSTOP. Flights August to December. Extended weekends available. Origin/Destination reversible. No Christmas dates available. Return flights before mid-September are “overnight Red-Eye” flights.
A Carry-on bag and also a personal item is allowed for free on American. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.