Adding Quantity discounts for Commerce items
Quantity discounts can be supplied for Commerce by going to the Commerce Module > Discounts tab and clicking Add a new Discount.
Discounts are comprised of two parts: the details of the discount, and the discount conditions. The details of the discount specify how the discount is applied, either as a percentage or a fixed amount; when the discount expires, which you can leave blank if it never expires; and how many times the discount should recur for items in an order that are recurring.
You can set as many conditions to a discount as needed, and the conditions can specify whether to include or exclude items that match the specified conditions in the discount; which group to apply the discount to; what category of items to apply the discount to; and what items to apply the discount to. Note that to apply a discount to multiple customer groups, you must specify multiple conditions for the discount. Each condition is specific to a group.
Conditions are entered from the Conditions subtab of the Discounts tab. To add a new Condition, click on the Add a New Condition button, the enter the condition name and a brief description of the condition. Click Save, then click Add a New Condition again and fill out the appropriate action, membership group, and catalog information.