I just took another look at the Frontier fares and fees. Yes, they charge $30 for carry-ons, but their checked luggage fee is in-line with AA ($25 for international flights). Plus they allow 1 free "personal item" such as a backpack or anything that can fit under the seat. With AA upping their air fares for PHL-MBJ to $790 for the entirety of the winter season, Frontier looks like it will be an attractive option. I'm just waiting for them post their fares for 2016. Seems odd that their price calendar only goes through December.