A lot of people say that Frontier Airlines is very strict when it comes to bags. The bag fees on Frontier Airlines mostly depend on how closely the airline checks your bag's size and weight. This guide from FlyAirlinesPolicy will tell you what you can expect at the airport.Frontier staff will look at the size of your items and bags at the gate. If your bag does not fit in the box, you will have to pay a fee for carry-on on the spot.