Flight1 is legit (although I never buy from them because of their stupidly confusing wrapper/client system). For some reason, flight sim addons seem to cause a lot of false positives with commercial antivirus software. I'm not sure why- maybe it's because they need to access weird registry files, maybe because the devs are amateurs and don't think to put in a bit of extra work to make them play nicely with AVs, etc. Now there have been cases of there being 'true positives' with flight sim software, even from otherwise reputable stores/devs. But I'm sure that your 727 for FS9 is fine, and flight1 is kinda useless but definitely not a malicious store.