Nintendo eShop Card codes are region specific. A Nintendo eShop Card bought for a US account can’t be used in a European Account. Please make sure you buy the card for the right region.
Nintendo eShop cards are the successor to the of the 7th generation. Instead of using a new currency like the points cards, they use virtual money, like the Xbox One and PlayStation 4.
Update: The $20 Steam Wallet and Nintendo eShop cards no longer work at PayPal, even though stock appears to still be available. With this, you are now forced to go with $100 denomination to get these deals. If any of the cards are expired above, just make sure your shopping cart is $75 or more, and the $15 coupon will work.