The “maximum premium” payable by any member of a mutual insurance company, other than a mutual life insurance company, shall be expressed in the policy, or in the application for the insurance if attached to the policy. Such maximum premium shall be a cash premium and an additional contingent premium not less than the cash premium, or may be solely a cash premium. No policy shall be issued for a cash premium without an additional contingent premium, unless the company has and maintains a surplus which is not less in amount than the minimum capital required of domestic stock insurance companies authorized to transact the same class or classes of insurance.