The Quantity Discounts add-on allows you to discount a product based on the number purchased by a customer.
Once you've installed and activate the Quantity Discounts add-on, you will see a Quantity Discounts option in the Advanced settings of your product editor.
To get started, check the Enable Quantity Discounts for this Product box then click the Add new discount button.
Then you can begin entering your quantity discounts. First you'll enter the number at which the discount begins, then enter the price that each item will be at that quantity or higher. You can create multiple quantity price breaks.
Once you've finished adding all of your quantity price breaks, be sure to save, publish or update your product.
Now, when a customer increases the quantity of the item they are purchasing, the discount will be reflected.