Book Store Choice between purchasing download ebook variation or shipped hardcopy.

Hi I am just new to membership so i don't know if this is the place to write the question...but i am using marketpress primarily to sell my ebook and hardcopy books...I couldn't find a way to offer two options to customers ie purchase as a download (no shipping, cheaper price) or purchase as a shipped hardcopy (shipping cost added) i tried doing the two as variations of the product (different prices) however when i uploaded the ebook via the digital download section of creating a product it came up with a message that no variations can be added to downloadable product. can someone please explain why this is and how i can get it to work so that customers can be offered to eighter download for free or pay extra and get the item shipped to them. at the moment i am having to create 1 product times twice 1 for the hardcopy version and one for the downloadable version. I am using the latest wordpress multisite by the way.

any help anyone could give me would be greatly appreciated.