Not my work:
Unison worked this out for buyers who have not yet received their GSP shipment.
TRACKING SYSTEM FOR GSP
I found this thread today while awaiting a shipment. Once I learned that Pitney Bowes was the relevant company, I was able to find my package's tracking info in about 10 seconds.
Step 1: Go to the Pitney Bowes Parcel Tracker page at https://parceltracking.pb.com/app/#/dashboard/
Step 2: Enter your eBay-supplied Global Tracking number (mine began with UPAA...)
Step 3: You will be shown limited tracking info. In my case, only up to the point the package left the USA and entered Canada.
Step 4: Click on the "Track on carrier's website" link at the top right.
Step 5: You will be provided with tracking on the page of the actual shipping company, in my case Canada Post, with a tracking number in THEIR system.
Problem solved! If only eBay would put a link to https://parceltracking.pb.com/app/#/dashboard/ in their shipping emails, this could all be avoided...