Usually everything in the PO is in large bags and just put in the PO van.  It would not be scanned until the bags were emptied at the sorting centre.

By rights they should be scanned into the sorting centre and then out when they are on their way to the next stop.

In reality they are often not scanned after the initial scan when they are lodged at the PO until the driver picks them up at the Delivery Centre in the area where they are to be delivered.

In other words, they are scanned at the beginning of their journey and again at the end when they are being delivered.  Any other scanning is a bonus and you cannot rely on it being done.