You can setup a Volume Discount for your customers from the Store / Discounts section of your admin account.
1) Hit Store
2) Click on Discounts
3) Now hit the + Volume Discount button to the top right.
4) Now work your way through the Volume Discount fields.
5) ..and hit Create when you're done!
..and you're Done!
Please note that Volume Discounts cannot be applied to any Package items in your customers' shopping carts.