The Maritime Administrator, acting for the Secretary of Transportation, shall have the right to change the rate of premium at any time, and unless the revised rate of premium is accepted in writing by the Assured within fifteen (15) days after receipt by the Assured of notice of the revised rate, the policy shall become null and void and of no effect as of midnight, Standard Time, at the location of the shipyard on the fifteenth (15th) day after receipt of said notice. Premium at the revised rate shall be payable for the fifteen (15) day period during which the insurance remained in force unless the Assured, within such period, dispatches notice to MARAD by fax, certified mail or courier of his refusal to accept such revised rate of premium, in which event premium at the revised rate shall be payable for that portion of the fifteen (15) day period prior to dispatch of such notice. Upon the dispatch of such notice of non-acceptance the insurance shall terminate.