Except for good cause shown, every order requiring payment of child support under this article shall require that the support be paid through the central unit for collection and disbursement of support payments designated pursuant to Kan. Stat. Ann. § 39-7,135, and amendments thereto. A written agreement between the parties to make direct child support payments to the obligee and not pay through the central unit shall constitute good cause, unless the court finds the agreement is not in the best interest of the child or children. The obligor shall file such written agreement with the court. The obligor shall maintain written evidence of the payment of the support obligation and, at least annually, shall provide such evidence to the court and the obligee.