The insurer may cancel any policy by giving the insured 20 days’ written notice of cancellation with or without tender of the excess of paid premium above the pro rata premium for the expired time, which excess, if not tendered, shall be refunded on demand. Notice of cancellation shall state that the excess premium (if not tendered) will be refunded on demand. The notice may be served in person or by mail, addressed either to the member’s last post office address or, if this is not known, to the address given upon the application which is part of the policy.