We recently found a great new service, originally we were sure it was a scam but after using it for a few months we can vouch for it’s legitimacy. Earny automatically scans your inbox and looks for invoices from retailers who do price matching guarantees in the following 30 days after purchase. We order a lot of hardware for our office and for our clients and ever since we signed up for Earny the checks have been slowly coming in, most are for small differences between $2 - $6 but a couple were for $40 and $75. The nice thing is that you can just enable it once and forget about it, and the only fee that Earny charges is a small percentage of what they manage to get for you.
You can signup through this link