I used JustFly to book an international ticket but only 18 hours later I found a cheaper price for the same itinerary. US federal law requires airlines to refund in full a cancelled ticket within 24 hours with no penalty but JustFly takes that down to just 4 hours, then they charge their own fee ($75 in my case).
I spoke with a customer service representative who stated the fee was charged by the airline (a lie) and that she could not cancel of change my ticket without charging the fee. I pointed out that the airlines aren't allowed to charge a fee within 24 hours and that the fee was therefore entirely at the discretion of JustFly. When I asked if she_could not_ or_would not_ decline to charge me the fee she said it was company policy and that nobody (yes, nobody) in the entire company could change or cancel a ticket without charging the fee (clearly another lie).
So, JustFly seems to be OK with selling a ticket but they'll gouge you with a private fee if you find a better deal within 24 hours, then to lie you and blame the airline for the charge.
I'll never do business with them again - if they lie about the fees where else are they being dishonest?