Coinfetch is described as 'Automatically get money back when prices drop on items you already buy online'. There are 2 alternatives to Coinfetch for iPhone and Online / Web-based. The best alternative is Paribus. It's not free, so if you're looking for a free alternative, you could try Earny.
Earny gets you money back on almost every purchase you make. Stores promise that if the price of an item you purchased goes on sale within 14 days of your purchase, they will refund you the difference. Major credit card issuers guarantee the same policy for up to 90 days.