(a) The board of trustees shall apply the amount of any employer contribution for optional coverages to the excess of the cost of the basic and optional coverages for which an individual eligible to participate in the group benefits program under Section 1551.101 or 1551.102 applies over the basic coverage contribution.
(b) Except as provided by Section 1551.309, if a participant applies for basic and optional coverages for which the cost exceeds the employer contributions for those coverages under this chapter, the participant shall authorize in a form and manner satisfactory to the board of trustees a deduction from the participant's monthly compensation or monthly annuity equal to the difference between:
(1) the cost of basic and optional coverages for which the participant applies; and
(2) the employer contributions for basic and optional coverages.