This may seem useful only to collectors but still, it's there. The Shovel Knight amiibo is fully usable while still sealed in its box. Watch this video to see it.
You can see that it's not completely automatic, and it takes a few repeated tries (though the first one works).
The reason Shovel Knight can be scanned but no others is because someone other than Nintendo made it. The packaging doesn't include a metal protective strip that prevents this kind of registering.
The amiibo has already launched in Europe, and hit shelves in North America just this Friday.
This video's maker, YouTuber NintenDaan, doesn't warranty that it will work with the Wii U GamePad's NFC receiver, but others have said it does.
Amiibo have been notoriously rare across-the-board, but some retailers allow you to search in store, and a scan of GameStop, for example, turned up a couple locations where it could be found. Of course, that assumes you can pull this off without a clerk noticing and coming over to tell you, hey, this ain't a library. The Skylanders toys don't have this kind of shielded packaging, though, and that doesn't seem to be a problem.
The Shovel Knight amiibo lists for $12.99. Some grey market listings on eBay go for more with the figure new in the box, but not for insane amounts. File this one under amiibo novelties, for now.