Hey Paul,
I am a musician and a web designer for musicians. I too have been looking for SquareSpace to add this to the store. As it is, it's a very clunky workaround. I tried Shopify - way too expensive, as they have too many "extensions" that cost extra. I don't like Shopify at all. ( yet another shop to connect to Squarespace).
I LOVE Squarespace, but not having the flexibility to sell Digital and physical products together is frustrating to my clients as well as myself.
Thanks.