Yes, and package delivery autonomous car can be made very small -- just big enough to carry the package. In fact, a ground-based autonomous delivery vehicle doesn't have to look anything like a car. Imaging an autonomous Segway-like 2-wheel cart. Segway is 4.5 times more efficient that Toyota Prius and 11 times more efficient than average American passenger vehicle.[1] Something like that would allow delivering packages very cheaply.
If everything is really well orchestrated logistically, your Segway solution could attach (piggyback) to a bigger autonomous vehicle for a certain duration for greater efficiency, higher speed and bigger radius. Of course, the fuel costs will have to be split, but that is just a matter of making smart accounting software.
I like that idea -- for less time-sensetive deliveries you could have a "mothership" truck loaded with mini-vehicles drive to a neighborhood center and then release them. However, for things like food delivery where you want to deliver in under an hour, it will be better to just use mini-vehicles.
[1] http://www.segway.com/downloads/pdfs/energy_efficient_segway...