I can't wait 'til Monday

I am going to the Post Office on Monday to ask them to explain a piece of mail I received today. 

The package was a cardboard mailing envelope, that had been sent to me by a company in Grand Rapids, Michigan. I have no idea what was in the package when it was mailed from Grand Rapids, because when it arrived in my mailbox - it was affixed with a label which read:

RECEIVED IN DAMAGED CONDITION (without contents)

The envelope looks like it had been run over by something. Three of the four sides were torn completely apart - and there was nothing inside.

I have no idea who sent me the package, nor have I a clue as to what may have been inside. But I am nearly 100% certain the person mailing it took it to the Post Office in much better condition than what was delivered to me.

The sticker from the USPS sounds like they have no idea what happened to the package - but if it was given to them in one piece, shouldn't they be responsible for the damage? And for the lost content? 

You'd think they would at the very least send the mailer a check for the postage. The sender paid the USPS to deliver the contents of the package to me. The PO failed in this and the sender shouldn't have to pay for services not rendered.