The charges fixed for the product supplied or the service rendered by any municipality shall be sufficient at least to bear all cost of maintenance and operation, to meet interest charges on the bonds and certificates issued on account thereof, and to permit the accumulation of a surplus or sinking fund to meet all unpaid bonds or certificates at maturity.
     The corporate authorities of any municipality owning and operating a municipal utility plant shall, in addition to fixing utility rates, have the power to establish a service charge for the late payment of rates charged.
     These amendatory Acts of 1971 and 1975 are not limits on any municipality which is a home rule unit.