There are no conflict between our products and the QW757.
Conflicts are not possible, since we wrote the installer for them, their installer gets the Addon Manager online automatically from exactly the same location (our server) so the installed files related to the Esellerate system are exactly the same.
This, of course, provided you *had* a working internet connection when you installed the QW757, otherwise a local version of the files is used, which means it's not certain you get the latest version but, if you *have* have internet connection available, it's just not possible ending up in a conflict between the two.
In any case, the method of purchasing doesn't have anything to do with this. Regardless if you decide to purchase on the Web store or directly inside Flight Sim, ALL our products requires the Addon Manager to RUN, and so the QW757 which means, the purchase method is not relevant, you'll need a working Addon Manager for both, even to run just the Trial version.
In any case, there are absolutely no problems running our products together with the QW757, the Addon Manager it's one and the same it handles both products, and they are both tested to be working together.
It's possible you had a corrupted download (or an unfinished download) for the Addon Manager files OR you failed to exclude the antivirus properly, which blocked the download. Now, since this is your first post, it would have been better if you asked a question here when you had the problem with KFLL, because we'd surely fixed it without requiring you to Uninstall it, which wasn't needed at all.
So, just download the JFK installer and you'll see it will work without any problems, provided you turn off the antivirus and configure it properly, as explained in lots of thread in this forum, for example here:http://www.fsdreamteam.com/forum/index.php?topic=4301.msg38948#msg38948