Tonight I went to the Ft Lauderdale airport to see if it was really true. I have been watching Southwest & Spirit for about a month for fares to Chicago. My kids are flying to see my ex June 11 - July 14. We usually use SWA so went there first. On line price $127 each way each kid (3) to Midway total $760.80. NO discount for airport - in fact alot higher. Then we went to Spirit. On line price was $118.78 each way each kid to O'Hare for total of $712.68. BUT if I purchased them at the airport, it was only $99.79 each! I had read it was the passenger use fee, but it was even more than that! So, on that flight, with 3 kids round trip, the total was $598.74! That is a savings of $113.94!! I don't know why since they charge for everything else, inclusive of printing out your boarding pass there - it is now $5.00 so they told me.
ANYWAY, it pays to read these postings and check it out. The airport is only 30 minutes away so it was well worth it.