Free shipping for specific products/categories with minimum subtotal

[ Moderator note: moved to Fixing WordPress. ]

Hi everyone, I have been going crazy trying to find a free solution to my requirement. I would like to offer free shipping when the sub total of specific products/category in basket is over a certain amount. So the customer will be charged for shipping when they ad items, however when a certain category of products have a subtotal over x amount, then shipping should be free for the entire order. Any help with this would be fantastic. Free plugin or code snippets I could use to achieve this with woocommerce?