Actually, Southwest doesn't offer nonstop flights from BOS to SEA- they don't serve BOS nor have nonstops from MHT or PVD. I'd check Orbitz.com (as most airlines are listed there) and www.AlaskaAir.com as Alaska Airlines is the only airline that flies nonstop BOS-SEA. Actually, I'd wait a while before booking tickets as it's still quite early and there will be some promotions, mile-deals, or fare sales sometime..