Cancelling Subscription / Uninstalling Multi Custom Authors

Given Billing is handled by Shopify, to cancel the subscription, simply uninstall the app and Shopify takes care of everything.

To uninstall the app, follow the instructions provided by Shopify here: https://help.shopify.com/en/manual/apps/uninstalling-apps

Once that's done, your subscription is effectively cancelled and Shopify will stop billing for you the app.