I’m assuming you already have setup the first doorbell and its associated with your protection plan. If not that would be the issue.
One more thing - make sure you Login to your account in the top right before adding to cart. Ring.com does NOT remember your login - you have to login in each time you go there. I always forget to do this and when you don’t login beforehand you don’t get the discount.
Otherwise contact support and ask them why your account doesn’t get the 10% discount.