The post-office was forced to add a bar-code to stamps precisely because there were a lot of forgeries.
I would guess that the bar-codes don't read properly when the stamp is put on a parcel, because unlike on a lettter, they were not flat, so the reader could not cope.
But the point is that the post-office took the money and must deliver the service. It does not matter if the stamps are fake or not.
It's just the sub-postmasters scandal in miniature.