There are several services like this (moolah, Paribus, etc.). They all require your email password and full access to your email account (they read your email), and some require your credit card or bank account info. When Earny finds price drops, they email the retailer on your behalf, pretending to be you, requesting a refund for the difference.
I’m not about to give any company full access to and control of my email account, not even LinkedIn for professional purposes. It’s probably best to pay for online purchases with a credit card that has a price protection program and/or gurantee. If you decide to use a service like Earny, create an email account that is used only for online shopping retailers and change your settings to forward all incoming emails to your primary email address.
http://www.cnbc.com/amp/2017/07/08/earny-a-retail-app-can-save-you-money–if-you-let-it-control-your-email.html
https://www.usatoday.com/story/tech/2018/04/09/cash-savings-refund-apps-can-help-bolster-your-bank-account/487628002/