Any person who voluntarily accepts responsibility under section 701¹ and agrees to pay his or her proportionate share, as determined under section 708,² of the response costs and damages listed in section 702,³ as ultimately determined, plus an appropriate premium in an amount up to 50% of that person's proportionate share, but not to exceed 15% of the total costs of response and damages, shall not be subject to claims by the department, or by any other responsible person, in excess of that amount.