Variations from schedule of rates

No common carrier or public utility governed by the provisions of this act shall, knowingly or willfully, charge, demand, collect or receive a greater or less compensation for the same class of service performed by it within the state, or for any service in connection therewith, than is specified in the printed schedules or classifications, including schedules of joint rates; or demand, collect or receive any rate, joint rate, toll, fare or charge not specified in such schedule or classification: Provided, That rates different from those specified in the printed schedule or classification of rates may be charged by any public utility, street or interurban railway, by agreement with the customer, in cases of charity, emergency, festivity or public entertainment: Provided, That any utility governed by the provisions of this act may grant to the officers, employees and agents of such utilities free or reduced rates or service upon like terms and in the same manner as is now provided by law relating to common carriers.