You don’t have to tweak the plugin. The filter I mentioned comes with eShop. My plugins just hook into it to decide whether UPS will or not be called.
A while ago I set up a plugin that can handle what you need. http://usestrict.net/2013/03/eshop-per-product-free-shipping/
With this plugin, all you have to do is set up a package class called ‘free’ and assign it to the Product or Product option that you don’t want shipping for.