Here's a news release from Airporter Shuttle offering a direct run once a day, Monday through Friday through the holiday season:
Beginning Nov 1, 2013, and through the holiday season, we are offering DIRECT SERVICE from Bellingham to Seattle & Sea-Tac Airport! One southbound run and one northbound run per day, Monday through Friday.
This service is available MONDAY thru FRIDAY only. No Saturdays or Sundays.
Southbound (Direct) Stops and Times
9:45am Ferndale WTA Park & Ride
10:00am Bellingham McDonald's
11:45am Seattle Convention Center*
12:15pm Sea-Tac Airport
Northbound (Direct) Stops and Times
2:00pm Sea-Tac Airport
2:25pm Seattle Convention Center*
4:15pm Bellingham McDonald's
4:30pm Ferndale WTA Park & Ride