We have yet to see how they would fly if they have to carry weights larger than their size. And if they cannot come up with a solution, then there is no point in simply being able to deliver shoe boxes only. If this service is to run, it should be able to transport every type of package.
Obstacles Drones are not allowed to fly above 400m. So, in a city with tall skyscrapers or in forests with tall trees, drones would not be able to function efficiently. In conclusion, using drones for delivery services is a pretty good idea.