My variation of this Automator workflow will:

• Detect when a Kindle is mounted.
• Confirms if you want to sync your device with Instapaper.
• Deletes any old Instapaper files on the device.
• Downloads a fresh Instapaper archive and places it on your Kindle.
• Asks if you want to dismount the Kindle, and does so if chosen.

Once you have this workflow in place all you really need to do is plug your Kindle in and wait for it to do it’s thing. Couldn’t be easier.

One issue to be aware of: while you have your Kindle mounted, any insertion of USB drives (or anything that will show up in /Volumes for that matter) will start the script. I’m looking for a workaround on that one, but it’s a relatively minor issue. Plug it in, sync it, unplug it. That’s the point of it, anyway.

You must also be signed into your Instapaper account through Safari for the Instapaper archive to be downloaded successfully. Automator uses your Safari session to grab the file.

Finally, the script should work with any number of Kindle devices, but only one may be plugged in at a time for the sync to work properly.

Download

You can download my workflow and customize it for yourself. You’ll want to move the file to your ~/Library/Workflows/Applications/Folder Actions directory and work with it from there. Alternatively, you can build the workflow manually.

Install the Workflow

1. If you downloaded the pre-built workflow, ensure it’s been copied to the location I mentioned.
2. Open Finder and press Control+Shift+G. In the prompt, enter “/Volumes” and press Enter.
3. If you don’t have the path bar enabled in Finder, go to the View menu and select “Show Path Bar.”
4. Right click “Volumes” in the path bar and select “Folder Actions Setup…”
5. Attach the “Update Kindle with Instapaper.workflow”, ensure both checkmarks are checked.

Plug in your Kindle, and watch the magic.

Build The Workflow Manually

1. Open Automator and create a new Folder Action. Set the “receives files and folders added to” drop down to “Volumes” (select Other, then press Control + Shift + G and enter “/Volumes”, press Go and then OK.)
    
2. Add a “Run AppleScript” action with the following code:

   on run {input, parameters}
   
   	tell application "Finder"
   		if exists disk "Kindle" then
   			return input
   		else
   			error -128
   		end if
   	end tell
   
   	return input
   end run

3. Add a “Ask for Confirmation” action. This will allow us to cancel our Instapaper update if we’re not ready for a fresh copy.
    
4. Add another “Run AppleScript” action to check for any existing Instapaper files on the Kindle, and move them to the trash for us.

   on run {input, parameters}
   
   	tell application "Finder"
   		set kindleFolder to "Kindle:documents:" as alias
   		set instapaperFiles to every file in kindleFolder where
   			name of it contains "Instapaper-ReadLater-"
   		move every item of instapaperFiles to trash
   
   		-- If you'd like to automatically empty your trash, uncomment:
   		--empty the trash
   	end tell
   
   	return input
   end run

   Lines 5 and 6 should be on one line.
    

5. Add a “Get Specified URLs” action. Delete the default URL and add http://www.instapaper.com/mobi.
    
6. Add a “Download URLs” action. Point the Where value to your Kindle/documents folder..
    
7. Add another “Ask for Confirmation” action. This time we’ll be asking whether we want to dismount our Kindle or not.
    
8. Add one more “Run AppleScript” action to dismount our Kindle from the file system, allowing us to just unplug the device without any further steps.

   on run {input, parameters}
   
   	tell application "Finder"
   		if exists disk "Kindle" then
   			try
   				eject "Kindle"
   			end try
   		end if
   	end tell
   
   	return input
   end run