American Airlines has round-trip flights from Dallas (DFW) to Cancun (CUN) for $246, NONSTOP. Flights February to May. Departures on Monday, Tuesday, and Sunday. Most departure availability is on Sunday however. Returns are mostly Tuesday through Friday.
A Carry-on bag and also a personal item is allowed for free on American. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.