is it possible for eshop codes to stack up?

because I wouldn't buy a $50 code just to download a $40 game. I really want to know if they'd stack up or not.

The money goes into your eShop account. So if you redeem a $50 code, you get $50 in your account. You can spend it whenever and however you want, and the leftover money stays in your account.

Pretty much what the guy above me said. So assuming you buy a 50 dollar card and purchase a 40 dollar game you will still have 10 dollars left. If you choose to buy another 50 dollar card and redeem it it will stack/add to the previous balance, meaning you would now have 60 dollars.