No other payment methods are allowed unless you have Shopify's agreement. You might want to reach out directly to Shopify - https://help.shopify.com/en/api/guides/charging-for-your-app/faq#can-i-use-paypal-other-as-my-billin...
The rule used to be applicable only to publicly listed apps, might not be relevant for private apps, but you might want to reach out to the billing team directly to confirm that is still the case.
|an hour ago|
|2 hours ago|