I fly Spirit all the time. They are very strict. If it can fit under the seat, you don't have to pay, but if it has to go into the overhead bin, they charge. They will not allow you to put anything in the overhead bin unless you paid the carry-on fee for that piece. Only passengers boarding in Zone 1 have purchased carry on baggage, and even when boarding is complete, the flight attendants will not allow you to put the bags under your seat in the overhead bin.
If there's any doubt your bag will fit under the seat, pay the $20 online or at check it because if it doesn't fit and you haven't boarded in zone 1, they will charge you $45
