Amazon seemed to have changed the URL format for item search. Please update the URL Template of the custom search to "https://www.amazon.com/s?k=@haste" (Change the part of "amazon.com" to the domain of Amazon in your country repectively.), or download the latest one(s) from the "Custom Search Downloads" → "Shopping and Auction" and replace the old with it.
In that case, reload the current page in Safari and try again. In particular, please note that Safari App Extensions do not seem to work on pages you opened before the extension was enabled. Please reload them and try again.
To remove the Safari Extension, delete ALL instances of "Haste.app" (or "Haste-beta.app" for beta versions) files from your computer and restart Safari. (It took us a while to figure this out!)