The commission shall, within 90 days after the completion of the hearings and arguments, if any, and the filing of briefs, if any, in any case submitted to it, or upon a rehearing thereof, decide said case, or if for any good and valid reason it shall be unable to decide the same within said period, then at the expiration of said 90-day period, the commission shall make and file and serve upon all the interested parties a written order stating its reason for its inability to decide said case within said time, and may extend the time for decision for a period not to exceed 90 days.