From the manufacturer

Use Nintendo eShop Cards as a fast, easy, and safe alternative to credit cards when purchasing games and other content including Fortnite on the Nintendo eShop for your Nintendo Switch, 3DS or Wii-U consoles.
After purchase, you will receive your download code by email with detailed instructions on how to redeem it. Your code can also be found on your Amazon account in your "Games and Software library".
The code can be gifted, you do not need to have an eShop account to buy this product.
You will receive the code immediately after purchase. In some exceptional cases, we may take up to 4 hours to process your order.
VALID ONLY FOR UK ESHOP ACCOUNTS

Redeemable on Nintendo Switch

How to use the code on Nintendo Switch:
Select “Redeem Code” or “Add Funds” in Nintendo eShop and follow the on-screen instructions.

Legal notices for Nintendo eShop prepaid code:
To use the code, you must have access to a wireless internet connection and accept the network-related terms and privacy policies. You may also be required to create or link a Nintendo Account.

This code:
* can only be redeemed once at its full value, for the same currency as used to purchase the code.
* cannot be resold, exchanged, refunded or otherwise redeemed for cash.
* will not be replaced by Nintendo or the retailer if lost, stolen or otherwise used without your permission.

Please keep the confirmation for future reference.
Issued by Nintendo of Europe GmbH.