Requirements:
1.) You must purchase a E-shop giftcard, doesn't matter from where (Don't worry if this doesn't work you can go ahead and sell it online or use it for yourself.

2.) You must contact them and say that you purchased an eshop gift card but you accidentally scratched off 3 or 4 characters to the point where it's unreadable

3.) They will ask you to provide a pic so go ahead and provide the picture, Make sure you wrote down the code before you scratched it off

4.) They will then send you a new one

5.) Go ahead and use both eshop gift cards on your account and you'll spend 100 and have two hundred, or you can sell them both and make profit

Note: You can only do this once per nintendo account (I've only tried it once for my account so you can go ahead and try)