If a one-time late payment charge is imposed, it shall not exceed an amount equal to 10 percent of the first $10 of any bill, plus three percent of any amount of such bill in excess of $10; provided, however, that a minimum charge may be made of not to exceed 15 cents per bill, and further provided that there may be allowed in any block of a rate schedule a late payment charge of one mill per kilowatt hour of electricity or one mill per 100 cubic feet of gas.