@nobody*s_perfect wrote:
USPS has special equipment at its sorting centers to detect underpaid (or overpaid) online postage and the difference is automatically charged (or credited) to the sender's account.
Ayup. That particular Priority Box has its own box ID barcode printed in four different locations (top face, bottom face, spine and one end), read by automatic scanners along the way, to verify that when that box barcode is seen, the Shipping label had better indicate that Priority Mail Flat Rate Small Box postage was paid. In this case only First Class Package postage was paid, so the seller will probably get dinged for the difference.
Fun fact: the barcode encoding for the boxes is the same as that of the tracking, so scanners can read either one, something I learned when trying to scan a package at the Self-Serve Kiosk after hours. It kept rejecting my label tracking barcode scan as "Invalid" for no apparent reason, until I finally realized that the way I was moving the box toward the scanner (which on the SSK is only about 18" above the floor), it was seeing the box ID barcode first, before I got the tracking barcode into view, and rejecting it. I finally noticed the error message telling me that "PS00011000003" was not a valid tracking number... which was true; it was the ID of the box. Holding the package label other-end-up worked better. 🙄
