Freeshipping it cart contains certain item(s)

I have read similar threads on this but I cannot find a solution to my needs... Most people want to NOT charge free shipping if the cart contains a certain item but I want to do the total opposite!

An example would be - a store contains products in categories A,B,C + D

If the customer goes to the checkout with items that only have products from B,C or D then they have the option of store pickup only. If they have any item from category A in their cart (either on it's own or mixed with products from the other catgeories) then they get FREE local delivery.

I have a rough idea on how to setup the zones so that delivery is only available in the local area (by adding new zones for the local areas and then deleting the others) but I cannot see how to get the option to show if category "A" products are in the cart?????

Thanks,

Tony