Originally Posted by
MileHighColorado
US airports only I looked into Freebird and it said it was only for US airports I’m booked on a frontier flight from Cancun to Denver
To be frank, UA's prices are roughly in range with F9 and it offers twice daily nonstop and one inconvenient connection via IAH. Nothing to be done if there is massive IRROPS at either station, but if it is a question of one aircraft, chances are that UA can scrounge up a replacement at DEN while F9 simply doesn't have the extra aircraft lying around.
Either way. Check your travel insurance to make certain that it covers not only the cost of a new ticket, but the possible loss of a prepaid vacation or at least extending the vacation for a lost day or two.