One of the first apps that would check for price declines and shipping delays was Paribus.co which is owned by Captial One Bank now. It works with over 25 major retailers including Amazon Target Walmart Nordstrom Gap and Macy's.
It monitors your email and watches for order confirmations from retailers it works with. When it detects potential saving or refunds you deserve according to the policies of the individual store it alerts you and helps you get your money back. The service is free to use.