The plugin is excellent and does precisely what it says it will. It is very easy to use and blends nicely with the content and appearance of my site. 5 stars!
But... just as a heads up for anyone having trouble... my affiliate account was created some time ago, and I was afflicted by what I can only describe as a bug in Amazon's program. This bug made this plugin, and all other Amazon affiliate plugins, seem as if it wasn't working properly. If you're having trouble getting this plugin to function, read on before blaming or abandoning it.
Every Amazon affiliate plugin that I could find was claiming my affiliate account was not registered with Amazon's product API. Including this one. Which is completely false. It was registered, and I was entering all the right information, in all the right fields, with proper permissions and accounts, and I still got this error.
In order to fix it, I had to create another affiliate account. Evidently, Amazon has been making tweaks to their backend and affiliate accounts that are causing headaches for some of us with older accounts.
Therefore, if you're having trouble, click the sign out button on your affiliate portal page (upper left hand side next to tracking IDs). This will redirect you to an affiliate login page... Click the sign up for a new account button. It will then walk you through the setup procedure. This new affiliate account will still be tied to the same Amazon account as your other one, but will have a new affiliate ID and will WORK!