UPS ships to Post Office ?????

I also work at the Post Office. you have no idea how many people send a package via UPS and they (UPS) drops the package off for us to deliver?
UPS pays the P/O to deliver the package. It makes good sense to UPS, they charge you $10 to ship a package that the P/O would have charged $7 to ship. We're coming to your house anyway.
Out of the 30 or so packages I deliver during a day, I'd say 5 or 6 are UPS referrals.
By the way, when you saw the UPS driver leave the packages, they weren't delivered until the next day. We don't come back to pick up the UPS stuff.
When the mail comes in in the morning, we take everything out. With the exception of some bulk mail (catalogs that are not delivery dated) we curtail nothing. Especially 1st and 2nd class and packages.
and inkjunkie is right, the P/O is a huge operation. there is a lot more going on than you would think.