So the clerk who is representing the merchant—who is basically a customer, in the sense that they pay interchange fees, which the banks receive and which you in turn receive a portion of—doesn't even know the price they're paying. It's as if you went to a gas station and there was no rate posted or no dollar amount on the metre, but you just filled up according to the number of litres on the gas pump and then walked in and just paid whatever they told you to pay, based on a rate that was not posted anywhere but was just a number after you'd already made the decision. That's what it's like for the merchant. When someone slaps down a card, there's no rate attached to that card that they can see. They just pay it.
On May 14th, 2009. See this statement in context.