How can I export books from Ubuntu to Amazon Kindle?
send them via email to your kindle account. Calibre has an option for that. If you are unfamiliar every kindle has a unique email that you can send documents to.,
You may try installing SendToKindle program written in python.
First install python setup tools:
sudo apt-get install python-setuptools
Then follow instructions on github repository:
https://github.com/miracle2k/sendtokindle#installation
After installation you should be able to send any file via right mouse click.