Motor common or contract carriers of passengers shall not be compelled to carry baggage of passengers, except hand baggage, the character, amount, and size of which the motor carrier may limit by its rules and regulations, subject to the approval of the commissioner; and the commissioner may by rule or regulation limit the amount of the liability of the motor carrier therefor. If a motor carrier shall elect to carry the personal baggage of passengers (other than hand baggage), the commissioner shall prescribe just and reasonable rates therefor and such other rules and regulations with respect thereto as may be reasonable and just, and may by rule or regulation limit the amount of the liability of the motor carrier therefor.