18. The Board may, upon its own initiative or at the request of any interested party and after investigation, amend the prices, rates and rentals demanded for his services by any owner of a public service, in such a manner as to render them fair and reasonable.
It shall be illegal for an owner to demand or receive prices, rates or rentals other than those of which he has produced the list or which have been fixed by an ordinance of the Board.
Any amount paid in excess of the prices fixed by the Board may be claimed back by the person who paid it or by his representatives or assigns, notwithstanding any agreement or stipulation to the contrary.
R. S. 1964, c. 229, s. 18.