I'm also getting this constantly, seemingly whenever I switch blocks.
It also appears in the text popups at the bottom of the screen (with all the drop notifications/etc).
I haven't purchased a single thing since October, and confirmed that in my purchase history (so it wasn't some hacker using my account or something). I've also never bought SG.
Edit: Also, just to make sure, I checked my Steam purchase history and my MS account's purchase history. Neither one has any recent purchases (MS not since last June, most recent Steam purchase was Valheim a few weeks ago, but obviously unrelated), so hopefully this isn't anything malicious and is just a weird game bug.