You are saying a postal employee did not charge the correct amount and shipped it? What if the receiving post office sent it back to you instead of charging the postage due?
I had printed the label at home and she was simply scanning it in. She wasn't concerned about charging me because I'd already paid for the label. She said if it did get caught along the way, the recipient would pay upon delivery (she doubted that would happen though as it was only going from Southern CA to Northern CA).