Glad you got it taken care of. I work for the USPS we have our share of bad employees but some things are out of our control. "flat rate boxes" are good but they aren't the strongest box on the planet an if you stuff 60 pounds of stuff in there they will rip open. With internet sales package volume has skyrocketed so the shipping containers are getting maxed out. They shove as much in them as they can. Also, we "share" assets with fedex an ups. I deliver parcels that are dropped off from ups an fedex everyday. they get them to the post office an we take them to the door. Also our priority mail flies on ups planes. Take a look you will see both a usps an ups or fedex lable on a lot of packages.
back to topic....glad it worked out sorry you had to deal with it.