Tangible and intangible products

I have used wp ecommerce many times and i really like using it, but i am having a problem with the checkout for a shop that sells both tangible and intangible goods. I don’t want the delivery details to show up for the intangible goods. I’ve spent ages looking for a way round it but i’m not seeing it. I can either hide the shipping details or show it.

I can add category specific fields to the checkout but i can’t seem to make it so that it only shows the shipping if it is in a certain category.

Am i missing something obvious?

thanks C ??