A debtor or secondary obligor may waive the right to notification of disposition of collateral under § 9-611 only by an agreement to that effect entered into and authenticated after default. A debtor may waive the right to require disposition of collateral under § 9-620(e) only by an agreement to that effect entered into and authenticated after default. Except in a consumer-goods transaction, a debtor or secondary obligor may waive the right to redeem collateral under § 9-623 only by an agreement to that effect entered into and authenticated after default.