Had the exact same experience with Just Fly. I bought a $145 ticket a month in advance to departure date. I needed to change a flight to a later date. Just Fly let me change the date for that ticket and it cost me $75 which was fine- and the terms were: I had to fly with the same airline as on the original ticket and if the new flight cost more I would pay the difference. Also I had one year to use it. Three months later I'm ready to book another flight using that ticket with the same airline only its $155 now. No problem, I knew I would have to pay any price difference. Just Fly CS was really cool and started booking my new flight told me I was all set and " there will be an additional $100 fee for booking". I'm like WTF? I already paid Just Fly $75 to change the flight. Just Fly CS said " we TOLD you it would be a $100 fee when you changed the flight 3 months ago". WRONG! I'm no math whiz but I know that $75 + $100 = $175 all of it Just Fly booking fees. Who would pay that for a $145 ticket that I booked MYSELF online!. And no where on my ticket or account does it say it will cost $ 100 fee on top of the $75 fee to change and book a flight.
Just Fly is JUST A TOTAL SCAM.
You'll pay double in "fees" than you will for your flight. And you'll never know about it until it's too late. They know how to run a scam. Use this company and you'll find out for yourself.