What is there to say? Amazon money goes in, Nintendo money comes out. Codes are generally emailed instantly (aside from one case where I had to wait a little while and got my code within a couple hours).
Protip: To deal with pesky cases where a game costs slightly more than your balance and adding another gift card would be wasteful, attempt to make the purchase anyway. On the screen where you're told you don't have enough money, there will be a button to press that will let you use a credit card to add the exact amount you need. I use this feature on the 3DS, but I believe the Wii U and Switch also have this feature.