Nintendo is changing how credit cards are used in the Wii U and 3DS eShops next month. It will no longer be possible to use credit cards directly in those eShops starting in September. Soon, users will need to visit a URL while having their Nintendo Network ID linked to a Nintendo account. Once that’s done, funds can be used in the Wii U and 3DS eShops as well as Nintendo’s site. As of now, these changes will affect EU eShops.
Instructions provided by Nintendo:
Visit https://ec.nintendo.com/my/balance_add, select the payment method you would like to use and follow the on-screen prompts to complete the process.
Once you have added funds to your NNID this way, you can use them both on the Nintendo website and directly within Nintendo eShop on your system.