title: Developer payload (external data) in App Store server notifications
body: <p>We are using server-to-server purchase notifications for Google Play and App Store mobile apps. User can pay for subscription in several ways: in web broswer (via third-party billing provider), in android application (via google play) and in ios/macos application (via app store). That is why we use custom user/subscription ids - it should be equal across all platforms/devices within single account.</p>
  
  <p>Now, everhing went just fine with other billing providers until we came to App Store. We configured server side notification with callback at our server as we did it before. And now it turns out, that there is no user information in App Store receipt data. And it seems to be no way to pass that data from mobile application. For example, Google Play have so called "developer payload" field for this purposes, other providers also have possibility to add external data into server notification request. Is there any analog for App Store notifications?</p>
  
  <p>Another question is about notifications itself. If there is no user information in the receipt - that means that there is no way to bind user id and receipt id data. Then what is the purpose of such notfications with external server scenario?</p>