I ran into what I think is the same problem. If you look at ZNodeShippingPackage there is a member variable _useGrossValue. It is declared as follows:

protected bool _useGrossValue = true;

This is what I think you want, except that the constructor changes it to false in the initializer list.

public ZNodeShippingPackage(ZNodeGenericCollection ShoppingCartItems)
: this(ShoppingCartItems, false, false) { }

I just changed the code to the following and I began to get shipping weight calculated using the total weight of all cart items instead of just a single item.

public ZNodeShippingPackage(ZNodeGenericCollection ShoppingCartItems)
: this(ShoppingCartItems, false, true) { }

- Rich