The direct number to USPS customer service is: 678-279-7980.
Be prepared for a 30-60 minute wait, but you will actually speak to a person.
I spent hours digging through amazon/ebay/etc seller forums, and eventually uncovered the phone number above. When I called, I waited about an hour to speak to someone, then the person at the other end added a comment to my lost package case number, asking the person assigned to the case to actually work on it, and my package reappeared in the system the next morning. YMMV but this worked for me.
Posting as a gist because gists seem to rank pretty highly on search engines, and it was way too hard to find this out.