Hi booklover6, and thanks.
I went ahead and took your suggestion and bought Calibre Companion -- the Amazon Coin payment system is actually kind of nice, I was able to get it free with my bonus 500 coins, and get some extra coins for my purchase. I got it set up and it works great.
I do have one question about Moon+ Pro. In the Amazon App Store for Moon+ Pro, it has a red "X" icon by my Kindle Fire HD 2013 from the drop-down list of devices (it has a green check mark for my Android device which is my Nexus 7). Despite that, I'm guessing I can still use this version of Moon+ Pro on my HD 2013?
Moon+ Pro on Amazon App store
http://www.amazon.com/Moon-Reader-Pro/dp/B004TN7QPG