(1) The posting at retail service stations of a different price for the same grade of gasoline dispensed from one pump than from another pump supplied from a common storage at the same service station when represented to be and is sold as the same quality of gasoline is unlawful.
(2) This section shall not be construed to prohibit a price differential between a cash sale and a credit sale of the same grade of gasoline or between self-service pumps and attendant-controlled pumps supplied from a common storage at the same service station.