That is odd, Rhody. I asked Frontier on Twitter and they stated that there was no fee for purchasing a ticket in the airport. When you say that the ticket was $6 more than booking online, were you using a promo code for the ticket online? Promo codes can't be applied at the airport, so sometimes with a good promo code it is still cheaper to book online.

Or did the ticket agent specifically say that there was a $25 fee to book? If so, I think you should contact Frontier and get your $25 refunded.