I was trying to set it up so my kid could order kindle books using amazon gift cards. Bottom line, at least in 2019, was that there was no way to do this. The only way to order kindle books was with 1-touch, which required a linked credit card. The system that I finally had to use was to send books to her account as gifts, and do the ordering myself. Also she didn't figure out how to use email until a couple of years later, so her account is linked to one of my own emails which meant that I was sending the books to myself, opening the gift email myself and accepting it so that it would go to her device. Here is the trace of the research that I did on this problem two years ago.
This thread seems to indicate that it is possible to use a gift card to buy a kindle book, but the account still has to have a linked credit card which was a non-starter for my case: https://www.mobileread.com/forums/showthread.php?t=116313
This YouTube vide seems to rely on some sort of glitch when setting up 1-click purchases and it didn't work for me. https://www.youtube.com/watch?v=nVA5d9-jXeg
There are a lot of hits for my question. The answers from 2010 don't seem to apply any more. There were other variations on the theme of the youtube video which was "set up 1-click but don't actually enter a credit card" so there must be something to it. https://www.google.com/search?q=amazon+kindle+no+credit+card&oq=amazon+kindle+no+credit+card
Lastly, because it's not at all clear how to send a kindle purchase as a gift, here's a page showing what it looks like to do it, followed by a sneaky advertisement for DRM removal software: https://www.epubor.com/how-to-gift-kindle-books.html
