Guy in a Prius actually costs more than a dedicated delivery truck for once a day delivery's. However, if you want same day delivery you are more or less forced to use the Prius model as you are stuck doing small batches of delivery's anyway.

Remember, the last mile cost for you is basically the cost from their last hop to you + the carrying your package to all the other stops. If the delivery guy is going to visit several houses on your street that's basically a few hundred feet on average.