As far as I can tell, this add-on does nothing useful whatsoever. Having not observed any of its 'benefits' during the past year, in which I've made only 6 or so purchases from Amazon, I have to assume this is a fairly useless app for the less-than-avid consumer, such as I.
It claims to show comparisons to "thousands of sites" but, this is Amazon's claim, not an independent arbiter's.