Any person, firm, copartnership or corporation doing business in the State of Mississippi and engaged in the sale or distribution of electricity, gas or water, whose rates or tariffs for such service contain any form of so-called “service charge,” shall be required to have fair and reasonable optional rate schedules and minimum charges that do not contain any form of such so-called “service charge,” so that the consumer may exercise his option as to the form of rate schedule under which said consumer will be billed for service used. Such optional rate schedules shall not require the payment for each unit of electricity, gas, or water consumed, at a rate in excess of the rates per unit prescribed for use which were in effect prior to the establishment of a “service charge” by such person, firm or corporation affected hereby.