I see plenty of cases on samsung.com. Tey this link: https://www.samsung.com/us/mobile/mobile-accessories/phones/?category_names=Phones&smartphone_compat...
That's different than the rewards page you get when you claim your reward. The $150 can only be used on the specific page you are given
Copy the code you are given. Then go to the Samsung shop app then select whatever you want to put in your cart. Then paste the code during checkout. The code should then apply the discount to the cases