to withhold from such employee’s compensation the employee’s share (if any) of premiums for health coverage (except that the amount so withheld may not exceed the maximum amount permitted to be withheld under
section 1673(b) of title 15), and to pay such share of premiums to the insurer, except that the Secretary may provide by regulation for appropriate circumstances under which an employer may withhold less than such employee’s share of such premiums.