I would like to understand how this works. Did they hand off the to-be-mailed uwrapped, say, lemon or a hammer or a deer tibia to a person at a post office? Wouldn't the post office clerks just say "sorry you gotta box that, we sell one of these here options"?

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?