Retrieves the URL for a Wix checkout page for the specified paid plan of your app.
This call succeeds only in case you have previously set up an external pricing page in your app's dashboard.
The returned checkout URL is valid for 48 hours.
This API allows your app to manage your pricing page outside of Wix while still using the standard Wix checkout flow.
You can only call this method when authenticated as a Wix app or Wix user identity.