Dear J2Commerce team,
 
I am using J2Store since many years for my webstore tamino-productions.com. I am selling musical scores as digital scores (PDF files) or print-on-demand (paper). In order to achieve this, I patched J2store to add an "options" tab similar to the Configurable Product to the Downloadable Product.
Here is the list of files I patched:
administrator/components/com_j2_store/views/product/tmpl/form_downloadable.php
components/com_j2_store/views/product/tmpl/adminitem_downloadable.phpcomponents/com_j2_store/views/product/tmpl/item_downloadable.php
The patch consists mostly in code snippets copy-pasted from Configurable Product template to Downloadable Product, and it appears to be working fine, for my website at least.
I can even prepare, test and submit a proper Pull Request on top of J2commerce 6 beta if that's more convenient for you. That would make the upgrade from J2Store 4 (patched) to J2Commerce 6 easier for me, but I think it might be useful for other users are well, giving them the ability to sell "configurable and downloadable" products.
Would you consider adding this modification to the official J2Commerce code please ?
with best regads
Patrick Loiseleur