This hasn't been discussed here in a while but was in the past. Buying tickets via Frontier.com has a $19 per segment fee for the website. If you go buy tickets at the airport counter, you can avoid this fee.

Some cautions:
1. You cannot use any promo codes if you buy at the airport
2. If you have to add luggage, it will cost more to do this by buying at the airport (in my case, it will be $38 for a carry on because I did this whereas had I booked on the website it would be $35).
3. You pay parking fee and waste time going to the airport

So chances are this isn't a great deal in most cases. In my case with fairly short notice and no promo code available it was worth doing.

I had to wait about 30 minutes to access the agent at the counter as well. Frontier only does 2 flights out of my local airport 4 or 5 days a week so the hours are kind of limited. I sure hope my flight makes it.