As someone who builds these for a hobby, the biggest problem is battery life and range.
The battery life life of the craft is determined by the weight/battery capacity ratio. Since is hardly any aerodynamics, the multicopter is constantly fighting the force of gravity by delivering an equal force using its motors and props.
An optimistic range estimate for current batteries would be about 10 mi, but the craft would also have to return, so make that 5mi.
Now imagine what happens when you attach a payload to it.
Regulations may change, and flight controlers keep getting better, but batteries would need significant improvement for this to become viable.
If range is a problem (and I have to assume that Bezos and team are aware of the performance characteristics of current/near term drone tech) they don't have to limit themselves to batteries.
The battery life life of the craft is determined by the weight/battery capacity ratio. Since is hardly any aerodynamics, the multicopter is constantly fighting the force of gravity by delivering an equal force using its motors and props.
An optimistic range estimate for current batteries would be about 10 mi, but the craft would also have to return, so make that 5mi.
Now imagine what happens when you attach a payload to it.
Regulations may change, and flight controlers keep getting better, but batteries would need significant improvement for this to become viable.