1. Deliver my package.
This is the online tracking page for the package in question. Notice how it made it all the way to Bothell, WA (where I live), but it was never actually delivered to me. Instead it was marked "Return to Sender" without my knowledge.

2. Explain why they can't deliver my package.
So I called my local post office looking for an explanation. After getting tossed around their phone tree a few times, the response I got was: generally, a package getting marked "return to sender" means there's something wrong with the address.
3. Help me figure out what went wrong.
That's alright I think, mistakes happen. However, when I then ask what the incorrect address was that caused the package to be marked "return to sender", nobody can answer me. I'm beginning to smell some BS. I also checked with the seller later, and they confirmed that they shipped it with the correct address.
4. Fix the mistake.
I don't really care whose fault it is, I just want my package delivered. So I ask them to correct the address and deliver it to me. The package is literally in the same town as me, so I figure this can't be that hard of a request. It turns out this is impossible. After getting escalated three times, the response I got was along the lines of: our system is not capable of doing such a thing.
Well, thanks post office, I hope you're happy. The sender is now sending me the package again, this time via Fedex.