Section 20. No physician or other provider of services, who treats or provides services to an individual covered by hospital, surgical, medical or catastrophic illness coverage offered by the commission under section four, four A, ten B, twelve or fifteen, shall charge to or collect from any insured or other beneficiary any amount in excess of that amount of compensation determined or allowed for a particular service by the insurer or by the administrator. For purposes of this section the words insurer and administrator shall include any insurance carrier, nonprofit hospital or medical service corporation or third-party health care administrator operating within the scope of its license. Nothing in this section shall be construed to prevent the collection of any copayments or deductibles allowed for in the plan design.