From the link:
> mailed at public postal collection boxes (when possible to cram the object through the aperture) or at postal stations (if possible).
In the US, post offices generally have drop boxes outside for letters (since you normally just need a stamp), and a larger drop box for packages inside, since you can often get pre-paid labels for stuff like item returns.
The article has lots of info about what clerks said and did at delivery, but little about when they were sent, which suggests to me that there's something about how USPS works that different from how it works where I live that's assumed to be obvious to the reader. You can't just drop a deer bone or a lemon in a mailbox right? It wouldn't fit, would it?