“Custom” Shopify apps (as opposed to “public” ones), often used for development purposes when developing Shopify apps, are unable to utilize the Billing API. Therefore, it is impossible to implement, test, and verify any functionality that deals with the RecurringApplicationCharge API. I feel this is a short-sighted choice on Shopify’s part.
I want to bring this to Shopify’s attention.
I also want to ask: How are we to actually implement, test, and verify integrations with the Billing API when developing an app?