That is your own speculation, not established precedent, and I would argue UPS is now acting as an agent of the government anyway.

So, again, what actual precedent exists to support rayiner's contention?

I agree. If you sign an agreement with UPS that allows it to simply give your package to the government upon its request, then you've effectively waived your Fourth Amendment rights.

But, absent that, the government cannot simply compel UPS to hand over the package or relay its contents without a warrant. That would be a violation of the Fourth.

OTOH, if the UPS employee simply looked in the box, it wouldn't be a Fourth issue. It is only so if the employee did so at the behest of the government. In other words, you are correct: in that case the employee is acting as a government agent[1], which triggers the Fourth:

[1] http://www.fletc.gov/training/programs/legal-division/podcas...

Edit: Key clause:

Miller: Can private parties ever trigger the 4th Amendment?

Solari: Yes, as we discussed, if a private party were to be acting at the behest of the government -- if a government agent were to ask that FedEx person to open up a package and look inside, or to ask someone’s girlfriend to go through their things looking for evidence to turn over to the police, then that would be government activity. That would be the actions of a government agent because government agents can’t ask private parties to do something they themselves couldn’t do under the 4th Amendment, so in that type of instance it would be extended to that private party.