This extension is unmaintained
I am personnaly using Alike03's extension which does what this extension used to do and much more.
Do not buy a game you already have access to
There is no way you're mistakenly buying a game again with the "Included with Xbox Game Pass" block appearing above the purchase options.
Review your wishlist
You might be waiting on a sale while you can already be playing games from your wishlist. With this extension you can tell at first glance.