Exclude shipping for a product

Is there any way to flag a product as not contributing to the shipping cost?

I have digital download products that are not flagged as digital downloads in Marketpress since it currently does nothing to protect the download URLs from non-buyers accessing them and only allows one digital download per order. I've implemented my own mechanisms for downloads and modified the order status page to provide the appropriate download links. However, these products cause a shipping amount to be added to the order. Is there any way to turn that off?