They have changed the way it shows. If you can't use a promo code on it, it only shows "Redeem any gift cards" when you go to the buy screen. If you can use a promo code on it, it shows "Redeem any gift cards or promo cards" and there will be a box when you click on that where you can enter the code.
Probably saves frustration for those who don't know there are codes.
You also can't tell for sure if you can use a code by the book being discounted already any more (i.e. it used to be if it had 2 prices you could likely use a promo code on it, now some that only have one price still allow promo codes).