I recently launch a side project selling software on Shopify and needed a way to for users to manage their software license. Shopify's sample Django app was out dated and didn't implement all of the security features so I've improved it and wrote a guide on how to set it up.
I've found Shopify to be a good platform for selling software minus a few things like subscriptions. It's nice to have most of the e-commerce features like discount codes, analytics, order management taken care of. The only limitation is their free subscription management app is very limited so you might need to pay or build an app to take care of that.
Hopefully, this should give you a head start on adding custom functionality to your next Shopify side project.