Way way way wayyyyyy old idea. Like 15+ years old. Remember Radio Shack? They had a scanner (was a cat or a mouse shaped device, I can't remember now) that hooked up to your computer. They gave it away free. I had dreams about this (and having scanners in a fridge, egg trays with sensors, etc.) years and years ago before the technology was so cheap and well connected to the internet. It will never be the device or scanner or voice control that is the reason for success.
The difference here is execution and value to the user. Not many people are actually in such a position. Amazon is. Amazon actually has something to deliver and they are now pushing on their local services which will make it difficult for companies like FavorDelivery and Postmates and then all the meal delivery services.
I agree, $79 is steep but they are able to test it out. I'm surprised they aren't giving away the devices. They are stupid cheap to manufacture. It's probably just a big experiment just like the Echo from Amazon. Speaking of, why can't you use voice with that to order?
So I don't think it's a gimmick. I just think it's Amazon trying to figure out what people want and what they are willing to pay for it. I do suspect Amazon will emerge with a very nice local service at the end of all these experiments.