I'm not in front of my computer so I can't query our fraud documents right now (I work in that business), but I recall quite some different schemes, a bit of everything:

Really forged labels, reusing old tracking numbers and that kind of things... Hijacking Mailer IDs and/or USPS API keys... And of course, hijacking accounts of USPS resellers. An e-commerce/retailer/whatever that uses an USPS reseller API for getting all kind of labels get hacked, and its API keys are used for generating a hundred thousand labels to be resold in eBay.