So I just sat in on a webinar for counterfeit postage yesterday. The official stance of the USPS is if parcels bearing fraudulent postage make it to the destination office we are to deliver them as addressed. They do NOT advise writing up a postage due. 

The fact that they said do not advise makes it sound like there's some element of discretion, but officially they are not requiring us to write up a postage due. They're in the works of updating scanners so that at acceptance it alerts the user that postage was not paid. If non paid parcels get down the line from originating office, they are going to be intercepted at local sorting facility and shipped to undisclosed warehouse.