[villagetinker]

Have not used Spirit, but Allegiant is an "alacart" airline, you get a personal carry on (must fit under the seat, then you pay extra for, any item in an overhead bin, your seat, any checked luggage (by weight), expedited seating, AND your boarding pass if you do not print at home, etc., I only use them when I want to fly into one specific airport, as Southwest (my favorite) does not go there.
Many airlines are going to this what I call alacart system, makes it very hard to get competitive of final pricing without actually making a reservation.