on the shipping tab, you probably have 2 shipping rules, one for USPS and a second for the Book and DVD set, both using the rate returned, i would think you only need the one rule.
the rate returned by USPS will depend on the setting you use in the USPS shipping server behavior on the confirm page.
Open the confirm page, then double click on the USPS Shipping server behavior to inspect the settings, on the last page, make sure that you have set the weight units to Ounces if that is what is being used in the cart.