Never, EVER buy a key from Kinguin. Those keys are usually volume activation keys that can eventually get deactivated, and then you're boned.
If you need a legit key, I have a Windows 7 Home Premium retail package (with no disc) that I think can still be used to activate a Windows 10 license. I will even toss in a USB drive with the 10 install on it if you want it.
Unfortunately I bit that bullet already. Only on the recommendation of a buddy who hasn't had issue so far. I got their 'protection' plan so we'll see how it works out. If it doesn't I'll certainly be contacting you.