delivery policies

I had a bit of a fright this morning. Received an E from amazon.com saying "USPS was unable to leave the package at your delivery address as per their delivery policies. The package will be available for pick up at your local post office. blabla".
I believe that by agreeing with the MyUs policies when signing up, I'd given them power of attorney (to sign on my behalf) so why would USPS make such a fuss about it? UPS didn't. I sent out mails to MyUS and amazon immediately trying to find out what's what.
Fortunately MyUs responded quite quickly, saying that's what amazon normally does when they find the business closed. USPS would reattempt the next working day so I shouldn't worry. Well, let's wait till tomorrow to see if that's the case.