United has round-trip flights from Newark/New York (EWR) to Lisbon (LIS) for $391, NONSTOP. Flights September to March. Departures on Monday, Tuesday, Wednesday, Thursday, and Sunday. No Fridays or Saturdays.
Carry-on bag is allowed without fee. Checked bag has additional fee. Seat selection fees can be avoided by accepting the randomly assigned seat during check-in.
TAP has inferior fares with expensive bag fees available on some of these dates.