Hello :-)
Today, I'm trying a new way to send books to my kindle. This idea will use three part, and almost everything is automated :
It's pretty simple

:
Firts, I use COPS send by mail feature to send an ebook to the IFTTT mail adresse with a specific tag in the subjet.
Then, IFTTT send the ebook itself in a specific folder of my Drobpox.
Finaly, Kindle-box see the file, convert it and send it to my Kindle on my kindle's adress.
But... there is always a but. Kindle-box can take a grat variety of formats (.epub, .mobi, .pdf, .azw) but not the .azw3. Sadely, all my book within Calibre exist in two formats only : .epub and .azw3.
So I have modified the
book.php file, and more specificaly the
public function GetMostInterestingDataToSendToKindle so it send epub in priority.
Then I've change the
$mail->Subject in
sendtomail.php to add the famous tag that trigger IFTTT.
And that work ! It requiere a little work at the begining, but using COPS from my NAS, so from any brower in the world, I can send my books to my Kindle, without having to have a third file's format to store. Basically, I could run with .epub only
PS :
There is one other way, without IFTTT on a Synology NAS : just add Dropbox in Cloud Sync, and then sync only the Kindle-box folder.
The user just have to take an epub book (or other format) and place it in the folder.