I would be inconceivable to me that someone would pick USPS packaging, pack my items in it and then deliver a seriously damaged package to USPS for delivery. Furthermore, I highly doubt that a USPS employee would accept a package with that much damage for shipment.
USPS on the far end may have very well received the package damaged but it was damaged within their system. A good precaution would be to take photos of your packages prior to shipment and preferably as you hand them over to the postal employee. It would resolve the dispute in short order.
Shipping heavy objects is particularly subject to damage due to the way that packages are handled. The last time I shipped 10 lbs of iron, it was in a plywood box made to fit the USPS box with the iron securely fastened within.