Tex. Ins. Code § 1452.207

An enrollee in the managed care plan is not responsible and shall be held harmless for the difference between in-network copayments paid by the enrollee to a therapeutic optometrist who is determined to be ineligible under Section 1452.206 and the managed care plan's charges for out-of-network services. The therapeutic optometrist and the therapeutic optometrist's professional practice may not charge the enrollee for any portion of the therapeutic optometrist's fee that is not paid or reimbursed by the enrollee's managed care plan.