You would need to write something to identify how many products are in the cart from that category and reduce the weight by that amount before the shipping quotes are made ...
There is nothing built into Zen Cart for this, other than to mark the products as Always Free Shipping if you mean for those to be Free Shipping regardless of the amount of the order ...
You could run an update on the products table for the always free shipping based on the master categories id, if that is the case ...



