It's not required, but, in most self-serve stations I'm familiar with (Costco being the exception), you can prepay with cash (e.g. ask the attendant for $20 on pump 5, or whatever), and the pump can be set to stop after $20 worth is dispensed. If less than $20 in gas fills your tank, you can get change from the attendant. This is commonly done late at night or for pumps not visible from the attendant's vantage point, to prevent drive-offs.