Shopify
Customers (not accounts) can provide all necessary data for the API to connect to their Shopify account.
This is not available for now, so a new table will be needed. One customer can have one or more Shopify accounts.
To be investigated
Is Shopify able to send push notifications?
Howto: REST call to Shopify every x minutes to check for an order? Or adjustable per Shopify account?