3. 1.1 Business: Payments - In-App Purchase
Guideline 3.1.1 - In-App Purchase
We noticed that your app contains a payment mechanism other than in-app purchase for digital content or to unlock features or functionality within your app, which is not appropriate for the App Store. In-app purchase is the only valid in-app payment mechanism for digital content.
Note: Continuing to hide functionality within your app or other dishonest acts may result in the removal of your apps from the App Store and termination of your Apple Developer Program membership and all associated memberships.
To resolve this issue, please remove all external or third-party payment mechanisms and implement in-app purchase to facilitate digital good transactions, including unlocking features or functionality within your app.
If you believe your use of an alternative payment mechanism is a permissible