That's quite a bizarre solution, but I suppose it could work. Imagine if it was widely used... The delivery driver could just go through business parking lots in the city and drop off packages into people's cars. No entering buildings, no leaving packages in the bushes or out in the open. It would be much more efficient and ought to make the packages more secure.

Of course, I can think of plenty of reasons why it wouldn't work, too. How do they avoid chasing your car around? What if the digital key system is insecure? What happens when thieves learn to watch the UPS truck and break into all the cars it places packages in?