USPS has........

I mailed something out at the Local UPS store. We were talking about lost mail, UPS and US Mail etc. He told me that the USPS uses UPS planes to overnight mail. I was surprised and he said, have you ever seen a USPS plane? Right now I am waiting for an ebay purchase being shipped by USPS, the tracking number has not been updated for four days, I am getting nervous that it is lost.