Free shipping is configured in the Shopify Shipping Settings. Once you have configured your free shipping, our Free Shipping Bar is used to promote your offers to your customers. Basically the app is an announcement bar that calculates how much has been added to the cart and subtracts this from the shipping goal to create progressive messaging.
Shopify does not have a shipping setting to exclude products from price-based shipping rates - the whole value of the cart is compared to your minimum order threshold. Therefore it is not possible to exclude products from a Free Shipping policy based on order value.
Note: It may be possible to exclude products using weight-based shipping rates. This can be done by manipulating product weights - for example, setting the weight to 0 for all products that receive free shipping, and a higher value for all products that need to be excluded. Free shipping would only be offered for orders with a weigh of 0, for example.
This approach is not recommended for a variety of reasons. It is much easier to simply add the cost of shipping to products you wanted to exclude, and offer free shipping on everything.
Please note that Shopify does not allow a minimum order amount for weight-based shipping, and it is therefore not compatible with our progressive messaging system. In this scenario, you would want to set the shipping goal for your bar to 0 and use it as a static message (something like: Free Shipping for $50 worth of hats - exceptions apply).