Mo. Rev. Stat. § 400.9-624: Waiver.

(a) A debtor or secondary obligor may waive the right to notification of disposition of collateral under section 400.9-611 only by an agreement to that effect entered into and authenticated after default.

(b) A debtor may waive the right to require disposition of collateral under section 400.9-620(e) only by an agreement to that effect entered into and authenticated after default.

(c) Except in a consumer-goods transaction, a debtor or secondary obligor may waive the right to redeem collateral under section 400.9-623 only by an agreement to that effect entered into and authenticated after default.