American Airlines and Delta have round-trip flights from Portland (PDX) to Cancun (CUN) for $321-$324 with a connection. Flights August, September, January, and February. Departures on Monday, Tuesday, Wednesday, and Sunday. Origin/Destination reversible.
A Carry-on bag and also a personal item is allowed for free. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.
Click for more details and booking links Cheap Flights: Portland to/from Cancun $321-$324 r/t [Aug-Sep, Jan-Feb] – American Airlines / Delta