I cant believe that there is no app for this simple yet very powerful feature. On Shopify app store , there are literally thousands of apps for similar kind of features, yet there is no app for this kind of function . Below is another non Shopify store with this feature , Its in another language but , you can understand the feature. It shows how much the final price will be at the checkout (green price) .It says %25 off for purchases over $300. and since the product qualify for the discount , it shows the price in green.
This feature make customer feel like they score a nice additional discount and encourage them to complete the purchase. Of course you can directly apply the discount but it wouldn't give the excitement of scoring a deal.