You could also pack a car full of explosives and send it on its way. It would be an undetectable 70mph cruise missile.
Or a truck. probably we'll find that vehicles left unattended will be subject to more stringent security. People will want to exercise control over the street in front of their house or business to prevent anonymous autonomous delivery. Vehicles driving about without supervision and/or improper credentials might be subject to arrest.
Considering how much trouble people have with malware on phones and PCs, there's probably some depth backlash possible if the gov't tries to backdoor everything.