The problem was the AWS key and secret key I was given to use by the client were incorrect. Once I got the correct keys, it works perfectly.
Very nice plugin. Love the amazon-elements shortcode. It adds a great deal of flexibility on what can be retrieved by the API.
The only wish list for me is adding a search tool by keyword(s), and another by ASIN. Keyword search could bring up a list of products, and then the product could be viewed on Amazon, or selected to insert into the post/page.
Just an idea... :)
Thank you for your work on this plugin!