A few months back though the Day One command line tool stopped working and that broke this action. But surprise!!! The command line tool works again (although not as well as it used to). So I modified the automator action to get it working again.
So, pop this workflow in your ~/Library/Services folder and you can just right click on the downloaded Amazon order history file to create a Day One entry from the purchases.
This is what the Markdown table looks like as a Day One entry (atypically expensive month, FWIW 🙂