if the carry meets their parameters it is free. pretty clear cut.. in fact my duffle bag was about 4 inches too long.. they didn't even check and it was free...
There was a thread on this airline before. Used them in January and I had no issues, actually loved it. $99 round trip from Dallas to San Fran. You don't have to pay for the overhead bin, as long as your carry is no larger than a gym bag, you are good to go. Don't ask if you bag fits their requirements or anything like that to get them to actually check your bag size or ask if you can use the over head bin, just use it, plenty of room up there. My bag was slightly bigger (about 1 inch on each dimension) no one actually checks to see if you meet the requirement on size. Drinks or snacks cost a few bucks, if you think you may need a snack, just grab something before hand. Prices are for things are about the price you would pay at a bar, 3-4 bucks for a beer, 2-3 bucks for chips or candy, etc. Print your boarding pass before you get to the airport or print it at check in before you get to gate/terminal, it will cost you extra to get them to print it before you board. It was $10 to pick your seats, me and my gf did that on the way to San Fran. On the flight back we were to find 2 open seats together and not half to pay. If you can pack light and prepare before you get to the airport, you should not have to deal with any of the extra charges. Will fly with them anytime I can.
I read somewhere the fee for booking inline through Spirit's website was like $7. Guess I was misled. For me, living an hour and a half from the airport, it makes no sense to drive in. It is more, though, if you book third party, as mentioned.