If you are flying Frontier Airlines, then avoid paying baggage fees by having a bag that is small enough to call a carry-on, but big enough to not fit in the small overhead bin in the aircraft.
Once you’re at the gate, Frontier Airlines will “gate-check” your bag by tagging it with a pink-slip for free. This way you don’t have to pay baggage fees. The downside to this is that your bag will be unloaded along with other checked-in bags and available to pick up at the baggage claim.
Also, passengers with a single carry-on that fits under the seat have priority to board the plane over others, so make sure to gate-check your bag as soon as you arrive at the gate.
Note that this may not be applicable on all Frontier Airlines.