Do you use Amazon? Then you must use Keepa. Not should. *Must.*
The amount of money I've saved from using Keepa has to be in the thousands now. Given the insane value it provides (not to mention the breadth of analysis and collation it performs in order to provide that value), it's borderline incomprehensible to me that it's a free-to-use service.
The only other browser extensions I've gotten comparable value from, and am equally thankful for, are ad suppression-related. If you use Amazon even infrequently, Keepa is a top 3 MVP browser addon, with a bullet. If you use it often, as I do, it's indispensable; by virtue of the amount of money I've saved using it alone, it's far and away the best.
I'll *definitely* be buying the Pro version and donating to the devs. Immensely grateful for this gem of a plugin.