If I pay with a credit card, I have to pay for the shipping. If I send a check, I pay the asking price or the auction result price. The gas is sold for a about 3 to 5 cents less per gallon if you pay in cash or with a gift card.
Yes, they have a different cost structure. In most states you can or actually have to pump your own gas. In New Jersey they require it to be pumped by a clerk. I know that from my years traveling the New Jersey Turnpike.
If a store accepts a credit card but doesn't give a cash discount, you can be sure they are charging all the customers extra to pay the card fees. They are certainly not eating that expense out of the goodness of their hearts. Such companies are forcing those who use cash to partially subsidize their card-using fellow customers. So those that do offer a discount are, in effect, charging the extra to those who use the service and not charging those who use cash. Would you avoid a restaurant that charged those who bought a hamburger less than those who bought a steak, or would you prefer they charge everyone the price of a steak, even if you only get a hamburger?
This same principle applies to stores who offer "no interest for a year" deals. They just include the interest up front in the price, so you are even paying it if you pay off the debt early. I am a retired economics teacher, and one day I was explaining it in class and using some local furniture store "sales" as an example. I told the class that if you asked about a cash-up-front price you could probably get 10% off. One of my students, whose dad ran one of those stores, said, "Our standard cash discount is 20%!" The foundational principle in economics is TINSTAAFL. ("There is no such thing as a free lunch.") Whether it is a gallon of gas at a station or a "free" government check in the mail, somebody has to pay for it some way.
I have had several no interest for a year offers in my life. The interest just gets added to the balance and you are just delaying paying it. Not much difference though. same principal.
The store initially lured me in with the fact that I could charge my purchase (thus putting of payment for 20 to 30 days). Now they want me to pay for the privilege. No thanks. There are plenty of places that will be glad to take my business and let me use a credit card at no additional charge. I will take my business there. How does that help the store that wants to charge me? They have now lost all my business over a mere 2% to 3%. If that makes sense to them fine with me! If their margins are so small that the 2 to 3% will hurt them, they will probably be out of business soon anyway.
If they're adding a 2-3% surcharge on credit card payments AND their marked prices are the same as the competition, then yeah, they're doing it wrong. They should be able to set their regular prices 2-3% lower.