Hi @treyfrancisco!
You can use Discoursify app for Shopify for that. DiscourseConnect option seems to be what you need: the login / signup of your Discourse community will be replaced with Shopify built-in authorization/registration system. Users will only need to sign up / log in once (to Shopify), Discourse will automatically get user’s state from the connected Shopify store. Plus, this way of setting up things is easy, you will find instructions in the app’s admin.