I am able to enter a shipping code with flat rate with the extra item addition. But, when I try to make a shipping code with extra item by weight (on calculated shipping), when I save the code, the extra weight addition does not save.

When I go back into the shipping code, only the original shipping weight has been saved, and the extra item weight is unchecked again. Creating a test auction shows that the auction calculates each additional item at the original shipping weight.

Accessing the site with:
Firefox 15.0.1, Linux x64