Nothing is more frustrating than buying a new pair of headphones, an OLED, or a backpack just to find out that you could have gotten it for a lot cheaper somewhere else. In order to keep customers happy — and prevent them from going elsewhere — many retailers offer price-matching policies where they promise to match a lower price that you’ve found elsewhere. That’s information that could come in handy, especially given Black Friday is right around the corner and Amazon’s Prime…