Ready to give up on shipping parts (or anything)...

We use both UPS and the USPS for the bulk of our shipments at our work. In most cases, our corporate UPS rate is cheaper than USPS.
We had a USPS shipment about a month ago that failed to deliver 1 out of 5 packages on the shipment. The customer has tried to trace through USPS.com without success and cannot get any reply from the USPS. As the shipper, we entered a claim for a lost package but the only response we've received in the past month is that they're tracking it.
Both the customer and us have been unable to get any voice contact or reply from the USPS.
Very frustrating.