(a) All persons who render services or provide things mentioned in §§ 11-9-508 — 11-9-516 shall submit the reasonableness of the charges to the Workers' Compensation Commission for its approval, and, when so approved, the charges shall be enforceable by the commission in the same manner as is provided for the enforcement of compensation payments.
(b) However, the provisions of this section relating to charges shall not apply where a written contract exists between the employer and the person who renders the service or furnishes the things.