I'm not terribly familiar with Frontier, but this fee looks to be identical to Spirit's Passenger Usage Fee of ~$18, in that it's charged for bookings on the website or via a call center. From brief searching, it looks like the fee has gone up quite a bit so far this year -- I've seen examples of $4 from January, and $10 from the spring, whereas I'm being quoted $18 for a flight this fall.

Can this charge be avoided by buying at the counter like it can on NK? I assume so, since the F9 site doesn't mention it being charged at the airport, but I haven't been able to find a whole lot on this fee.