I have successfully shipped two PC (Windows) games built using JMonkey
via Amazon.
- it doesn't cost anything to upload a game
- additional SDKs aren't necessary
Amazon have an Indie Game Store where you can elect to list your game.
From this page, there are two buttons where you can Sign Up and Submit your app.
https://developer.amazon.com/public/solutions/platforms/mac-pc
After building my JMonkey Desktop Applications (properties >
Application > Desktop), I created-and-added a README.txt file to the
foo-Windows.zip file that reads like this:
README.txt
My application requires Java: http://java.com/en/download/ ==== To install just extract this zip file: - right click - select: Extract all - click Next a few times on the Extraction Wizard ==== To run: - double click FooBar.exe ==== To uninstall simply delete this directory. ==== The object of the game is ... brief description ... Google+ community: https://plus.google.com/u/0/communities/105623681261195604330 IRC: irc.freenode.net #logFarm em: foo@bar.com This game was created using the *free* JMonkeyEngine: http://jmonkeyengine.org/ Thank you for playing: ....
adding the README.text file to the foo-Windows.zip file via Ubuntu was
simple, just double-click/open foo-Windows.zip and drag-n-drop the
readme file onto it.
Click the Submit Button to publish your application (zip file) to
Amazon and start clicking check boxes, etc to go through the
submission process.
graphical assets are required for the application’s Amazon web page:
- screen shots
- a large banner image
- a large icon
- optionally, a video
That’s it.
It takes a day-or-two for new submissions to be processed by Amazon.
Note:
Amazon currently only sell/distribute Personal Computer software
to the United States.
Here’s the explanation they gave me when asked why:
We appreciate your inquiry. As of now the Developer Portal only deals with US content. This is why you are unable to see your content in the UK or anywhere else for that matter. We are currently working through a whole slue of legal regulations regarding what type of content is appropriate and legal to submit to all or any other market places. We do plan on having the feature of what market places you the vendor would like to sell in, in the future roll out of our next developer portal.
Also note:
You are not allowed to add URLs to the description of your
application, the description that will appear on your application’s
Amazon web page so: ‘http://www.jmonkeyengine.org’ is not allowed.
Your submission will get rejected until you remove them. I fell foul
of this.
I have also published Android versions of these games, created with
JMonkey, on both Amazon and Google Play.
Regarding MacOS
It would be trivial to also publish the MacOS versions of these games
on Amazon but I have no way of testing the MacOS versions so I
haven’t.
Here are the links to the PC versions on Amazon.com:
Hoping this inspires others to publish their JMonkey game and if
anyone else has experience with publishing their games on Steam or any
where else, it would be great to hear what their experience has been.
Best regards
(edit)
In the end, because I am from the UK Amazon had some issues and so these applications are no longer available on Amazon 
They are available from my web page if you are interested (just to prove they do exist):
Bubbles in a cage
Cube Cracker