On 3/23, I sent 5 books packages with preprinted postage. They were all dropped in a collection box outside my local post office.   Four of those packages have safely made it to their new homes.  The 5th package still only shows - Electronic Shipping Info Received with the date and time.  Is there any way that I can get help from the post office to find out what may have happened to my package?   I have nothing to prove that I really did mail the book?  

You can call the 1800 number.

However, the barcode is only for delivery confirmation, not tracking, so there might not be any scans until the package is actually delivered. There is no sure way to prove that you mailed the package, but on PBS that isn't such a big concern because credits are only exchanged when the book is received (or earlier with credit confirmation.)

Hope this helps.

USPS.com has a tracking application.  Enter your DC number, they sometimes have more information than filters through to PBS.