Didn't see this posted yet.

Spirit will no long allow you to book flights at the airport any time their counter is staffed. They have instituted "Preferred Ticketing Hours," ostensibly to discourage folks from taking advantage of booking at the airport in order to avoid PUFs.

I was turned away from DTW at 8am with no line (two folks checking in with 3 agents) and told to come back between noon and 3pm, 7 days a week. I'm sure it's not a coincidence that those hours are impossible for anybody with a normal job. Fortunately for me, I have nothing better to do but drive to the airport to save a couple bucks.

In Spirit's defense, this was posted on their website and I was unaware nonetheless. I got into with their Twitter staff and the hours are, in fact, not preferred, but mandatory:

Our ticket counter does not only assist guests who wants to purchase tickets. They also assist guest who wants to check-in and any concerns about their flights on that day. That's why each airport counter has prefered ticketing hours. As mentioned, DTW's preferred ticketing hours is from 12 noon to 3:00 pm (Daily). As suggested, you cannot book flights outside those hours especially those times that our airport counter is busy. The best times to purchase a ticket at the airport are during the preferred ticketing hours. Even during the preferred ticketing hours, we can’t guarantee that there won’t be a long wait due to the unpredictable nature of the airport. The quickest and easiest way to book is on [Sprint's website]. I apologize for the inconvenience this might have caused you.