Isotope Product Download / Physical

I'm setting up a store that sells audio files.

They can either be purchased as downloads or as physical CDs. Is there any way to do this using variants so that each product doesn't have to be created twice?

I've set up a custom attribute 'Type' which has two options 'Download' / 'CD'. The problem is when I select 'Exempt from shipping' as an attribute option it's ignored in the wizard. This means I can't check 'Exempt from shipping' for specific variants and customers are asked to fill in their shipping details for the downloadable version.

I need to be able to switch 'Exempt from shipping' on for the 'Download' variant.

Does anyone know how I can do this? Or how I can achieve something similar?

Thanks