Yes, I know AS metal does not fly to PHL, but it codeshares on American Eagle on BOS-PHL.

SEA-PHL $168 AS
Fare basis: GCE14NU

Day/Time restriction:
APPLIES FROM 12:00 N. MON THRU 12:00 N. THU. APPLIES 12:00 N. - 11:59 P.M. SAT
Minimum Stay: If travelling Sunday - Friday, then Friday stay required. If travelling on Saturday, then Saturday stay required.

The only routing possible is SEA-AS-BOS-AS-PHL with BOS-PHL under an AS code.

AS SEA-BOS timetable:
AS12 SEA-BOS DEP 0855 ARR 1705
AS15 BOS-SEA DEP 1755 ARR 2105
Because of these times, this effectively reduces the allowable travel dates to:

SEA-PHL: Valid only Tues - Thurs
PHL-SEA: Valid Mon - Wed, Saturday
SEA-BOS-PHL prices out to $200 after taxes.

Miscellaneous notes:
- The change fee for this fare is $50.00 (compared to $100 on other airlines)
- This fare is not bookable on Alaskaair.com
- One could use the AS coded SEA-BOS codeshare on AA (AS1529); this would involve a forced layover in BOS (Arrive 21:56, fly BOS-PHL next day at 18:15). Reminder: forced layovers cannot be booked online.