(b) An appropriate public authority may increase the dollar amounts for copayments, deductibles, tiered provider network copayments and other cost-sharing plan design features; provided that, for subscribers enrolled in a non-Medicare plan, such features do not exceed plan design features offered by the commission pursuant to section 4 or 4A of chapter 32A in a non-Medicare plan with the largest subscriber enrollment and, for subscribers enrolled in a Medicare plan under section 18A, such features do not exceed plan design features offered by the commission pursuant to section 4 or 4A of chapter 32A in a Medicare plan with the largest subscriber enrollment; provided, however, that the public authority need only satisfy the requirements of subsection (a) of section 21 the first time changes are implemented pursuant to this section; and provided, further that the public authority meet its obligations under subsections (b) to (h), inclusive, of section 21 each time an increase to a plan design feature is proposed. Nothing herein shall prohibit an appropriate public authority from including in its health plans higher copayments, deductibles or tiered provider network copayments or other plan design features than those authorized by this section; provided, however, such higher copayments, deductibles, tiered provider network copayments and other plan design features may be included only after the governmental unit has satisfied any bargaining obligations pursuant to section 19 or chapter 150E.