In essence, you would like to encourage people to purchase more by illustrating potential rewards.
In general, the case with all e-commerce websites is that the main focus is always given to the product. Since you wish make potential rewards your main focus, you can provide an icon towards the bottom right of the product, where on hover, the user will be able to see current cart amount plus the potential benefit either in % reduction or comparison between otherwise amount & benefit amount.
Below is an illustration of the concept. Your product listing is same as traditional e-commerce website but benefits are communicated via a strong colored icon. This concept is for product list page where space is always limited. On the actual product page, you can try different iteration of this concept. I hope it helps.