In order to promote freedom of choice by employers and others in Delaware who purchase group health-care coverage, it shall be unlawful for any insurer, health service corporation or other person in the business of providing or insuring health-care services or coverage, to offer any insurance or health-care coverage to any person in this State on a basis which would preclude such person from allowing some members of a group to elect to enroll in a certified managed care organization, either by means of an express prohibition or by requiring the same payment regardless of such election; provided, however, that it shall not be unlawful for such persons to offer insurance or coverage on a basis where the rates or cost thereof are calculated according to the number of persons in the group for which such coverage is provided. The Department shall have authority to enforce this section.