There’s also:
It configures a Shopify store to be an OAuth2 identity provider. On the Discourse end, you’d need to configure the OAuth2 Basic plugin with the credentials provided by the Shopify app. It’s a fairly expensive app ($2,689/year for 2500 unique users.)
If you’re sure that you only want users to be able to login to your Discourse site via Shopify, a Shopify app that used DiscourseConnect instead of OAuth2 might be the way to go. The limitation with DiscourseConnect is that when it’s enabled it becomes the only available login method for the Discourse site, so you couldn’t have some users login via DiscourseConnect and other users login with a username/password.
For the sake of completeness, there’s also this: Single sign on with Shopify Multipass - #17 by angus. Take note of the warning in the Github repo’s readme though: GitHub - paviliondev/auth0-shopify-discourse: Integrate Auth0, Shopify and Discourse.