In TX's site's where to buy section, Germany's 2nd store works. Never bought from them but since it's in the official where to buy section I don't think it would be a scam. And I don't think it matters where you buy from since it's an online activation key.
Note: If your Switch is v1 (Erista) Patched model, you can run atmosphere in it without the need for a license.
Can't find any threads about this particular issue on GBAtemp or the wider intarwebz. Saw one video on YouTube that seemed relevant, but it was some dude with the same issue, no solutions offered in the comments. :/