App Store
Signed by AppleApple signs each notification with a certificate chain, so there is no secret on either side and nothing to leak or rotate. The first notification that verifies records your bundle id, and everything after it has to match.
Paste the ingest URL into App Store Connect as your production server URL.
Nothing on until you turn it on
Waiting to be turned on: SUBSCRIBED, DID_FAIL_TO_RENEW, EXPIRED, REFUND.