In my webshop I have free shipping as default.

Now I want to introduce a new category of articles and I want the buyer to pay a fixed shipping fee whenever he buys one or more items of this category.

I also have a fast shipping fee. When buyers choose it they pay a fixed amount, regardless of the quantity of products.

So, I want the shipping to remain free for all categories but this new one, and I want the buyer to pay the same fee regardless of the quantity of products, unless he chooses the fast delivery shipping, where he'll pay a different value.

How can I go about this?