Shopify

Shopify tells 404 Emails about new and updated customers through a signed webhook. 404 Emails never asks for access to your store.

1. In 404 Emails, publish a form under Signup forms - customers join through it.
2. In your Shopify admin, go to Settings → Apps and sales channels → Develop apps, create an app, and add webhook subscriptions for customers/create and customers/update pointing to the Webhook URL shown on Settings → Integrations → Shopify. Copy the app's signing secret.
3. Create an API key with the View and submit signup forms permission on the API keys page (or choose an existing one on the Shopify page).
4. Under Connect on the Shopify page, enter your shop domain and the webhook signing secret, choose the form and the key, and save.
5. Create a test customer who accepts email marketing, then check the page says Connected.

Only customers who accepted email marketing in Shopify are subscribed.