Add a Shopping portlet to your page.
Add an item, and set the Stock to 20.
Set the prices like it is in the attachment (Shopping_Pr3.PNG).
Sign out, and sign in as a user.
Find the item, and click on it, then on Add to Shopping Cart button. You can see the result in attachment (Shopping_Pr4.PNG).
On the next screen, change the quantity to 7. You should get 2 x 2.5 = $5.0 discount, and the price should be $65.0. But unfortunately, the Subtotal is $70.00.
Other bug is that the Shipping cost is $0.00 instead of $6.00. (See in the attachment: Shopping_Pr5.PNG.)