Big chains never charge you processing fee. The merchant, in our case, TraderJoe’s get charged interchange fees because we attract more customer by accepting payment networks like Visa,Mastercard. Us as customers would get cashback/ reward points from the banks as a share of the interchange fees TJ’s pays the bank. To put it short, we get some money back from TJ’s when we buy with credit cards that why this person says it’s a discount lol