"Priority Mail" is an euphamism (sp!) - It is POSTAGE only and is a marketing ploy dreamed up years ago to try to compete with FedEx/UPS/DHL - it is simply what used to be known as "First Class"... for more dinero!... there is no tracking number issued with just PRIORITY level postage...
To get any kind of "tracking number" on a package requires you to add an extra level of service, such as Certified, Insured, Confirmation of Delivery (a phreaking joke if ya ask me!), Signature Required, Registered or Express Mail.
If the shipper told you there was a tracking number associated with your shipment, it means you paid for miore than just Priority Mail... Now, what extra level did the shipper provide?
Otherwise, you should only be expecting a simple package that is absolutely untraceable (ever tried to track a stamp???)... and could be delayed by weather, by miss-routing, by about a million other little things (I run the Office Services for a law firm - you wouldn't believe the mistakes we recieve EVERY day!)
In fact, we sent a Priority labeled envelope from Seattle to Bellevue... a matter of a few miles and a bridge across the lake... It took 3 weeks!!!! Via Connecticut!!! When the PO scans the address and paints a barcode on your shipment... that barcode could actually be the code that the machine scanned for the previous item... Now your item has the address painted on by machine for somewhere else entirely!!! And until a human pulls it out and corrects the mistake, it stays in the wrong place...
But, considering the sheer volume the PO handles every day, well, I'm suprised that they don't screw up more than they do...
If you want to be able to track a shipment through the USPS system, only Registered or Express mail gives you the same tracking abilities as you find with UPS or Fed Ex. The other services are only entered into the system at time of mailing and time of delivery...and that isn't considered to be "tracking".
And no, I don't work for the PO, nor do I use them for my own personal shipping, as I like the way Fed Ex does things and the comfort level I've developed by using them over the past twenty years... But there is a lot of confusion out in the world about how the PO works.. or doesn't in some cases...