At MCO, one of my recent airport purchases of a one-way ticket (booked in end of January) took a good 10 minutes of agent time. Spirit has a new reservation system implemented in a few airports where after a purchase is made, the agent has to use another system to print the receipt. Unlike a basic login to my spirit account where a lot of information auto fills, the agent has to manually type in the information (like my name, etc.) which consumes a lot of time.
I wonder how much agent time would I consume there if I was booking for a family of four?
And, when I wanted to check-in for this flight, I got an error that I couldn't because I was a minor. Agent didn't fill in DOB correctly. I had to call Spirit for another agent to fix this issue so that I can check in online.
In general, I felt that I was taking up a lot of agent time, when most behind me just wanted to check in a bag for a same day flight. At ATL, the agents might be using the old system. However, even there, it still takes a good amount of agent time, especially noticeable when I'm there and people are behind me just wanting to check a bag.
At ACY which is a small station, I had a poor experience where I stand in a line, then I tell the agent that I want to book a new ticket, and I'm redirected to stand in another line with people there. At MCO and ATL, the agents haven't done that to me. ACY also has limited hours to sell tickets (up to 6:30pm I think), even if there are Spirit flights departing after that.
At PHL, Spirit operates at Terminal A away from the other LCCs (that operate in Terminal E) that attract their type of passenger. If I'm heading to PHL to fly Frontier or Southwest, but want to book another ticket on Spirit for a future date, it's a hassle.
Overall, I suspect Spirit offers discounted tickets at the airport to prevent fare matching as much. But wouldn't it make more sense to just email discount codes (e.g. 15OFF)? I'd think it causes more operational problems selling at the airport.
AFAIK, the FlightFlex add-on option is not available for tickets that are purchased at the airport.