What worked for me on one shipment with an extremely vocal, angry customer was reporting it as Lost Mail. Opening an investigative case doesn't seem to do anything except a call from the final post office saying they don't have it there.

After two weeks, I reported it lost and to ship back to me if found. What do you know? It magically appeared at the sorting facility that night and was delivered the next morning (to the customer). Not saying it's guaranteed to work, so YMMV.