How can I make free shipping overriding other products in cart? Setting priority?

First of all I have to confess that I am using freeshipper module to do something slightly different. I have some large products that need special shipment, I have added a shipping cost.
This works very good as long as the client only wants that product.

When you add some other products in the cart, they override the free shipping, thus my extra shipping cost for large items dissapear.

How to make Zencart checkout to prioritize freeshipper above the normal shipment costs?

(Even if I wouldn´t have messed arround with this module - in fact you might want to set it up so that when you order one item with free shipping, you would get the entire cart with free shipping.)