With the implementation of this feature, you can create a tiered discount promotion on the basis of the number of items your customer has put in the cart or the total shopping cart amount of your customer. Create up to three tiers of discounts and discount rules for your store. For example, buy for $100 and get a discount of 10%, buy for $200 and get a discount of 20%, and so on. As and when your customers reach one tier they’ll be notified with a congratulations popup. They can choose to continue shopping to reach the next tier or they can check out right after getting the first tier discount. 

https://apps.shopify.com/automatic-discount-rules