We cooperate with a shipping company which brings all your packages sent via USPS or Planet Mail to USPS sorting center. That is the reason for this special tracking number. Once USPS sorting centre process your package they will assing you classic USPS tracking number. Anyway original tracking number with PLANET in the beginning will still works. Thanks to this process we can offer you one of the best pricing on the market. See the example below which shows complete tracking of package with PLANET tracking number.
It is just on you if you will track your package using by PLANET tracking number or USPS tracking number. Both work the same. You can find tracking of your package directly in your account or you can also use universal tracking websites like 17track.net.