Keepa for Chrome, Firefox, Edge, and Opera provides price history charts for millions of Amazon products and can alert you when a price drops.
Keepa will watch any Amazon product for you and alert you by email, web push, telegram, or RSS when the price drops. You will need to log in with a user name and password to take advantage of this feature.
You can set up the desired price, new or used, show the item on Amazon, and delete tracking at any time.
Keepa also keeps a list of great deals online, so if you don't need alerts, you could shop for deals anytime.
Opera users will need to install Install Chrome Extensions, then download from the Google Chrome link.