If you purchased the black Deluxe Set of the Wii U, one of the promised perks was 10% back on all your eShop purchases. In order to take advantage, you need to go here to Nintendo's sign up page and login with your Nintendo ID. You'll then automatically be registered in the program, and all purchases you've already made will be credited to your account.
The promotion runs until December 2014, so you'll get 10% back on purchases for the next two years. Points can be redeemed in $5 increments, 500 points = $5 eShop credit. Points must be redeemed by March 2015.