As I understand Amazon shipping, Usually only the last leg of the Amazon shipment is done via the USPS. That's because it is cheaper for Amazon.
As far as shipping goes, we ordered a bathroom/sink vanity from Bed, Bath and Beyond/Overstock. Half the order reached us. The other half has a UPS label pending for 3 weeks! Lots of text on the BbB/overstock order site, but no explanation of why it seems to be stuck. Called today, and they re-ordered it - all of it - and ask that we send back the piece we already have. Very inefficient, right? Now I have to watch to make sure we're not billed twice.
So it's not just USPS.