I just did a Google search on PID numbers:
https://www.google.com/search?q=PID+tracking+number&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&channel=sb

A lot written about it. It seems that you can use that PID number sometimes to get free tracking results, and some times it does not work (!?!?). Frankly, I don't need free tracking from a Post Office clerk because I use PayPal labels with free tracking.

Here is a funny YouTube video on the subject of free tracking:
https://www.youtube.com/watch?v=jgZQ_1Gixng

Here is an eBay message board discussion:
http://community.ebay.com/t5/Shipping-Returns/USPS-Package-Identification-Number-PID/td-p/17198649

Robert