Amazon changed some things at the beginning of the year, they made it very difficult to get the actual file in azw format. They only let you download kindle unlimited books in their secure kfx(?) format, which current DRM removal plugins are unable to process.
If you buy the book you can goto Content & Devices and download the book in azw3 format which can be processed by the DRM removal plugin.
From what I’ve read amazon is monitoring the sites where they’re developing kfx bypass mechanisms and are sealing up those holes before a public release can be made. Which is irritating because I just hate the Kindle app and prefer MoonReader
Every Larian game so far was exactly like this, they always did early access for cheaper then went full price when it came out of early access.
BG3 was full price from the start, probably because of the money they had to pay WotC for licensing