Alaska Airlines will be the only airline with daily nonstop flights to and from Seattle (SEA) and Pittsburgh (PIT) beginning Sept 6, 2017. This service is daily but there's only one flight out to and from. Introductory fares are $350, including taxes and fees.
Price comparison: Other major airlines have this fare, with one stop for about $330, including Frontier which have fare restrictions.
Travel Availability: September 6 thru October 16
Use those dates and plug into Priceline [priceline.com]
, Orbitz [orbitz.com]
, CheapOAir [cheapoair.com]
or Expedia [expedia.com]
. Bundle hotel with airfare to save money.