Delay shipping label email until carrier first scan

Currently the shipping label email is sent the moment a label is generated. This creates a "tracking stall": the customer receives a tracking number that shows no movement for an extended period, because the carrier hasn't physically scanned the parcel yet.

The request is to trigger the shipping label email only once the parcel has actually been received by the carrier (first scan / handover event), rather than at label creation. This way, every tracking number shared with the end customer is live from the moment they see it.