If you can't get it to work, try this:
Add $50 or more of eligible products sold and shipped by Amazon.com to your shopping card.
Go to your payment options and choose your linked AMEX card with Membership Rewards (must have a card linked already)
pay for at least a portion of your order with Membership Rewards points (can just use one point) (click choose different amount to do this)
Enter the promo code AMEX16EVER in the box provided on the checkout page
$25 of savings will be reflected on the final order checkout page.