Section 6. The rates to be charged by an insurer for an insurance certificate contract under an insured legal services plan approved under the provisions of this chapter shall be filed and determined in accordance with the provisions of sections one to five, inclusive, and six to twenty, inclusive, of chapter one hundred and seventy-five A. The rates to be charged by an insurer under a group insurance policy for legal services benefits shall be filed and determined in accordance with such provisions of chapter one hundred and seventy-five A; provided, however, that such filing and determination procedure shall not apply to any insurer which maintains at all times an unassigned surplus of not less than twenty million dollars.