Glad you got what you wanted.
Having steered you towards "Send to Device" with success I can now admit that I rarely use that menu option directly myself any more
. I use the Reading list plugin mentioned above instead. I just add stuff to my default list for my Kindle whenever I have the need, such as after editing book content or adding yet another "must read" book on there. The Kindle is usually not plugged in at the time. But the list is setup so that the next time I do all those new/changed books get copied to it, and I also have it updating a tag so I can always see in Calibre what is on my Kindle regardless of being connected or not.