I don't know what the modern rules are. I first encountered these machines in the mid-1990s when the obvious expectation is they are not connected to any form of network and you wouldn't be able to reconcile it now with the USPS as required a network connection then it should be trivial to require a unique barcode for everything and they can verify the barcode really exists but I suspect they haven't obviously did all the old machines that aren't network connected and so there's no way to verify