UPS...what's the deal?

I'm confused on UPS shipments to P.O. Boxes also. UPS says they do not deliver to P.O. Boxes. I asked the local postmistress about this and she said if UPS brings in a package for a box they will accept it and leave a note in the box.

On eBay I see a lot of address such as:
Mary Doe
P.O. Box XXX
XXX Main St
Anytown, XXXXX

That way if it enters the system as USPS, it goes to the box. If UPS they have a physical address.

I suspect every physical address in the U.S. is in a UPS datebase somewhere and they use a computer program to determine whether or not to delivery to the residence or the Post Office.

They have a cooperative agreement here... UPS still delivers the big un's. But the small book size packages from UPS get redirected into the USPS system. I've been at the PO when the UPS truck pulls up and drops off piles of little stuff.

What's strange, is recently I had three packages coming from the same source on the same day, two biggies and a little un.... they were split up!

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For years UPS has been delivering Goverment checks (SS, SSI, Veterans, State) to the local PO.

I order my business checks from a third party printer and UPS delivers the box to the local PO and it is delivered by the letter carrier. This has been happening for at least 4 years.