Choose the product you want to track the price of, go to the shopping option on Google. Turn on the "Save" button against the name of the product, it will turn blue Go to the product page, you'll see a price tracking option, enable it. For the products I track the prices of, it notifies me whenever the price drops across websites, for example in the pictures above, I turned it on on the Styledotty page, but I track some Bath and Body Works products, and whenever there is a price drop, be it on the BBW website, Myntra, Amazon, or Nykaa, I get a notification. Edit- I should mention though, that I do not usually get notification for nominal price drops, like 10 or 15%. It's when it goes on a good deal like 40-50%, that's when I get them. More on reddit.com

If you use chrome there's a button at the top of the window to price track and get notified of changes. More on reddit.com

There is already a service like this called 'Honey' More on reddit.com

Use a third-party list and IFTTT. I use Todoist to achieve this. You can also use Life360 to track each others' locations and set up alerts when either of you is at the grocery store. More on reddit.com