Thanks to a comment on this site under a post about profitable side projects (which I wish I could find again), which pointed me in the direction of Shopify and it's app store about 3 years ago. I created Simple Purchase Orders https://apps.shopify.com/simple-po and haven't looked back. Coming from developing apps for Google and Apple app stores, they are a lot better at engaging with developers, it's not all good though, I'm currently trying to get them to remove a fake review maliciously left by a developer of a rival app.