USPS tracking is worthless. They almost never bother to scan the packages, and often they haven't even created a racking page until the box in question has been delivered. Huzzah for UPS: cheapest and best!
Being someone who has frequently shipped several books purchased form me off Amazon.com, ALL the ones with tracking numbers have been promptly updated each day.
The USPS number I have is also 22 digits, not the normal 20, and I tried removing the last two but that didn't work either.