If you use credit, most stations don't charge extra. Some stations will charge eight cents a gallon or so for the "credit price" though (but you know it before you pay), and will give you the "cash price" for debit but will charge the transaction fee of 35-25 cents.
That eight cents per gallon IS charging extra. Sure you know it before you pay but it's still extra above and beyond the cash price. What Mirlyn is saying is they use their card and get a DISCOUNT off the cash price. A lot of stations here give you 3cents/litre off if you prepay at the pumps with your credit card.