Small robots that can deliver small packages from one location to another on air and ground. They are tracked through an encrypted interface.
Operators operate robots remotely, share code and data for further improvement of courier AI and credited through bitcoin micropayment system.
Bigger payments incentivize courier operators to pick up object and deliever it to another location where other operators can pick it up.
Obviously, something need to be figured out to prevent operators engage in robotic combat for packages, not get into traffic accidents, not get package stolen, and so on.
This will happen, imo. Sooner or later.
Protecting packages won't be an issue. If the technology exists to allow a robot to navigate through a complex urban field, then it will almost certainly have also some equipments to track/watch the package and prevent theft. Also, nobody has to know how valuable is the package, and therefore random theft won't be lucrative.
Small flying drones already exist. What we would need now is enough AI to allow those drones to fly from a address to another, while avoiding obstacles and such. I think it will happen, but not in this century.