It's pretty easy to setup Kindle for PC to work with calibre. Assuming it is the only device on your account, all purchases will automatically queue to download into K4PC (if not, set your default device to K4PC).
As you buy books, they will download to K4PC whenever you open the application.
You MUST use the win32 version, not the Metro app. You can download the right version at
http://amzn.com/kindlepcdownload/
Then, set your calibre auto-add folder (Preferences ==> Adding books ==> Automatic Adding) to the "My Kindle Content" folder.
Whenever calibre is opened, it will import your new books.
If you have the plugin by Apprentice Alf (
https://apprenticealf.wordpress.com) then the books will be automatically DeDRMed as well,.