Are you asking what could they do or should they do? Or what is legal? Or what is allowed by the merchant agreements for a business accepting credit cards? I think that they cannot charge extra for using a credit card (or debit card) but they can arrange it so the normal price is what credit card users pay and app users and Teslas with a card on the account get a discount.
I think it might be a legal issue if they charged, for example, GM cars extra. I don't even know how they would know that.
I think part of the reason the government money is tied to universal access (and possible kiosk with display) is so that you know the price before you start charging.