I use this to cut through the bullshit of Prime Day deals or Black Friday. It shows you the historical price of a product.
Http://keepa.com is also really good
I literally just used this site last week to set up a price alert for a purchase.
Noticed via their chart that the item I wanted to buy (an air purifier) went on sale every few weeks from $340 to $199. I set the alert for $200, and the next day it emailed me saying the price had dropped. Saved me 140 bucks which was awesome.
Works for Amazon.ca links as well.
Crazy how I have a bunch of alerts set up on here, prime day started and I didn’t have a single item drop in price.
Also I believe there is a browser plug-in for this as well to make it even easier to use.