The rates payable to the undertakers in respect of any goods shipped or unshipped within the limits of the harbour, dock, or pier shall be paid as follows; (that is to say,) if such goods are to be shipped they shall be paid before the shipment, or if such goods are to be unshipped they shall be paid before the removal of the goods from the premises of the undertakers, and before the expiration of two months next after they were unshipped.