[jdmichal]

Cash has transaction costs too. You have to store the cash in a local safe. You have to pay for counting and accounting of the cash. You have to pay for safe transport of cash from onsite to offsite. You have to pay for trust, because disappeared cash is generally unrecoverable.

As long as there is no difference between the cash price and card price, it's rational for a customer to use a card with rewards. And I have never seen anything outside of gas with differential pricing. And you know what? I just avoid those gas stations as much as possible, because I can pay a similar price at another station and get my cash back.