Cheaper Spirit flights at the airport?

The following answer from Spirit Airlines (extracted from the thread you linked to) seems to answer your question fully.

Dear Mr......

Thank you for contacting Spirit Airlines.

Fares purchased at the airport are not assessed the Passenger Usage Fee. As a result, it is possible the total fare may be less at the airport.

Please note, however, that our agents do not have access to web-based or $9 Fare Club fares.

Please feel free to contact me with any further questions or concerns.

Sincerely,

Mark 62605 Corporate Consumer Relations Spirit Airlines, Inc.

So it seems that the fare in your example would indeed cost $16.11 at the airport, to the extent that it is available at all. Do note the bit about agents not having access to some fares, it means you can't always predict what the price will be with a subtraction. Either the same fare is available and that's the right computation or it's not and all bets are off.

For additional background for people who may be looking to do this, I recently purchased tickets using this same method on Frontier Airlines (which has a similar charge), and came across two complications.

1. If you need to purchase baggage, realize you'll lose some of the savings, as you cannot purchase it at the lowest price (typically $30 for the first right now on the majority), because to get those savings it must be purchased online as you purchase your flight... and so is thus mutually exclusive to purchasing at the airport. I saved ~$40 total for my two tickets, but paid $8 markup on the one bag I needed (it costs more if you don't add it to your booking online within 24 hours). (Whatever you do, don't have the desk agent add it, as that was to cost $50) (Keep in mind that for all the budget airlines, all checked bags cost... as do carry ons, but the terminology leaves something hidden that some may not know... they have a separate class called personal items, which are laptop bags and such - items that can fit under the seat (they have specific definitions of what size they can be) - of which 1 is free. Still unless you're absolutely the most absolute bare-bones of traveler, you may well need a bigger bag to hold clothes and such])
2. For Frontier at least, they sometimes have sales that use codes to bring about additional price reductions. However, the codes may be marked as online only... and so once again, it's mutually exclusive to buying at the airport. I lost another $9 total on this ($4.50 per ticket).

It still proved beneficial to buy at the airport for me. That said, those are some additional complications that may undermine it for many. In addition, despite reading it back to me (there was no kiosk I could find at KMCO, so I had to go through the line and do it through the desk agent), they still messed up my date of birth at booking, so be very critical that they get the right information exactly (there is a significant fee to change your name, though changing the birthdate quickly did not incur the fee thankfully). You do get 24 hours to cancel any reservations in the US for free, so theoretically if that happened you could cancel and rebook (though perhaps wait to cancel until you get to the airport, so at least there's less danger of the fare subsequently disappearing), but that's certainly a big hassle and expense. So make extra-sure they spell your names correctly.

One thing I seemed to find in my research on the flights was that the low-cost airlines do travel consisting of two legs differently: it appeared Spirit offered connecting flights with the doubled fee as Carl suggests (so you get about $40 back for a two part flight), Frontier only adds the cost once (so you'll only save $20 per ticket on the listed price [minus the differences mentioned above]), and Allegiant doesn't offer non-direct flights to be selected (so you have to book each one separately and thus save the $20 per flight). That can also be useful on the aggregator websites when considering how much the flight will end up costing using the airport booking method.

So it does work, and it does offer savings. But understand there are also some complications that some, such as a single tourist traveling alone, it may not be cost effective in the end. And you definitely have to be on guard through the whole process. So just throw it in as yet another complexity to trying to work together the cheapest travel option for your trip, but every little bit can help!

I went to Myrtle Beach airport this morning to purchase non stop spirit tickets for Dec 2, 2017 to Dec 10, 2017 from Myrtle Beach to Fort Lauderdale, the price was the same online and at the airport, all the years I have bought tickets at the airport they were always cheaper. Reading this letter tells me people at the airport have no of seeing passenger usage fee, what can I do they look at me funny at airport