We can't directly restrict a shipping method on a category, currenctly what can be done is enable the option "Use price per product" in the shipping method parameters and then in the desired product edition page set the price to 0 to set it as free.
HI!
Free shipping is based on dimension or weight.
Create a new shipping method with zero $ and set it free if is over .. for example .. 4 kg and set the weight at your products you want to send free < 4kg. So the method of free shipping will appear for those products. play around with this settings and will go thru for sure.
