Cheaper Frontier flights at the airport?

As confirmed at One Mile at a Time and FlyerTalk, both of which are broadly reliable for such information, you can indeed avoid the booking charge (CIC) by purchasing the ticket at the airport ticket counter.

Although Frontier's contract of carriage asserts a right to assess an "additional processing fee" for tickets purchased at an airport counter, its Travel Policies guide excludes airport purchases:

Carrier Interface Charge: the standard fare price we display online includes a charge per passenger, per segment, that is assessed on tickets purchased through the website or our call center. The Discount Den fares we display online include a charge per passenger, per segment, that is assessed on tickets purchased through the website.

That is for good reason. As Travis writes,

Frontier, like Spirit and Allegiant, categorizes part of the cost of their tickets as a convenience fee for using their website. That allows them to avoid paying the federal excise tax on that component of the ticket price. But in order for it to be exempt from this tax, it has to be an optional fee, which based on how other ultra low cost carriers handle ticketing, suggests Frontier tickets purchased at the airport may not be subject to the fee.

That said, reports are that as with basically everything else with ultra-low cost carriers, they make things as difficult for you as they can at the airport, too. First of all, consider these warnings from FT user storewanderer in a more recent thread:

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.

Furthermore, the staff may not be well-trained, and may not even be aware tickets can be sold at the counter. Frontier's "At the Airport" states that ticket counters are open from 2 hours prior to departure time until 45 minutes prior to departure time, which seems like a pretty narrow window to work with, especially since agents must prioritize helping passengers check in before they handle other requests.