Provide free shipping if a sample product is purchased

Hi
I’m building a website, but the plugin may not deliver as I expected.
I would like a feature where the customer gets free shipping if a sample of a product is purchased.
It works fine if I create a product as a sample, and give it the shipping class that provides free shipping. The problem is that it does not work if I give the product the same shipping class under the plugin’s tab under product data.
 
(I have the Pro version of the plugin)