Sadly, Amazon has stopped refunding customers when a price drops – so it's even more important to keep tabs on prices when they change.
Paribus has a beautiful app and website. The company offers refunds for non-Amazon sites (BestBuy, J. Crew, Target, Sephora, and others), and will also track Amazon Prime deliveries that arrive late, contact Amazon for you, and can get a $10 credit for your account. (BTW, Paribus is a free service that we like, but Paribus compensates us when you sign up for Paribus using the links we provided.)
Keepa doesn't have as sleek of an interface, but it's free and can also track price changes so you don't buy something right when it's just gone up.