Comment by lainwashere

I ended up using this to download all the azw3 files for my kindle, it's probably not the best you could do, so feel free to use it as reference for yourself if you might do something similar.

  curl -s https://standardebooks.org/opds/all | grep -oE "/(.*).azw3" | sed -e"s/^/https:\/\/standardebooks.org/" | xargs -n 1 curl -O

P.S. Calibre seems to only work with them and the kindle when you send over USB (I keep my paperwhite in airplane mode, signed out, ads disabled)[0]

[0]https://news.ycombinator.com/item?id=20596300