I'm not an expert in OTAs, but a couple of things...
JustFly appears to be a pretty sketchy operation. There's
a thread on FT. CBC did
a story about the BBB investigating them. There's a lengthy
thread on TripAdvisor about it.
Commonly reported problems include:
- You're flat out charged more than they quote you
- They make errors in the booking - e.g. misspell your name, wrong dates, etc - then charge you big fees to change them
- They attempt to charge ridiculous fees for making voluntary changes
I would stay away for those reasons alone. But looking at your specific example on EWR-PEK, AC.com doesn't show me the option you say it wants to charge $5795 for. I can bring it up through
Google Flights, but Google Flights shows booking through AC as the cheapest option by a few hundred bucks (and the link to book direct with AC works fine).
So stay away from JustFly. As for the other one, seems montrealer has a pretty good response.