Every express company doing business in this State which shall demand or receive a greater compensation than that which may be prescribed and fixed by said Commission for the transportation within this State of any class or kind of property, money, papers, packages or things, shall be deemed guilty of extortion, and shall forfeit and pay to the State of Texas a sum not to exceed five hundred dollars for each offense. If it shall appear that such violation was not wilful, said company shall have ten days to refund such overcharges or damages, in which case the penalty shall not be incurred. The said Commission shall have authority and it shall be its duty to sue for the same in such manner as may be prescribed by law for like suits against railroad companies.