I found this site, that some of you may find useful: https://www.packagetrackr.com
It allows you to track your packages, and doesn't seem to have a problem with Tor, although javascript is required.
But, if you're looking for a missing package, and don't want to set off red flags by tracking it directly and you haven't signed up for Informed Delivery, this service may prove useful.
Although, for extra precaution, I would make sure to only track from a Whonix Workstation, just in case the javascript tries to deanonymize you.
Edit For the love of god, do not track your packages on USPS.com, and block commenters that recommend that you do. You will not only get yourself arrested, you will get the vendor and all their other customers arrested too. You will get your package and address flagged, then the package will be profiled, then everyone is screwed.
Edit2 Since, people obviously aren't getting this. Allow me to make it more simple. PackageTrackr is a FREE proxy, that you can use with Tor, so you don't need to trust them, and it keeps USPS from knowing YOU tracked your package.
Nice LEO website. Tryna get my tracking numbers to intercept my packs nigga?