Hi @loki
Welcome to the Auth0 Community.
There maybe a couple of options for you.
If it was easier for you to use Shopify as your IdP (your user base is there but you can login to your Auth0 apps with those accounts) you could use our integration https://marketplace.auth0.com/integrations/shopify-social-connection also see here https://community.auth0.com/t/create-shopify-connections-for-customers/91149
More specifically to your scenario, you may prefer to keep Auth0 as your IdP in which case once a user is created/signed up in Auth0, you can configure a Post User Registration Action where you let Shopify know a user has been created/signed up and send the user_id across as a link and have Shopify create the user at their end with the Auth0 Id sent for mapping/linking purposes, Shopify can then use that account for orders etc.
Would any of these options work for you?
Warm regards.