Two things:
Wouldn't every single item in the store need to be connected to the internet to verify the transaction? I know internet of things is getting popular, but that would be one hell of a cost. The follow-up thought I have is well, instead of attaching the internet to the device, put it in a container that only opens once the transaction goes through. And then I realize that I've brainstormed up a vending machine. So my question is: how come vending machines aren't used for more items? The interaction is essentially the same to what you're describing: you walk in, interact with a thing, receive product, and walk off.
Second thing: you wouldn't be able to get rid of cash registers until the vast majority of customers were using bitcoin. Yes, the cost of the system might be offset by not needing cashiers/cash registers, but it wouldn't offset the cost of losing 95% (on the low end) of your customers.
tl;dr Many negative things. Sorry
For a couple pennies I would imagine something like a small radio chip could communicate with the change indicator. When you send BTC to a QR code or something like it, it's the device you're using that's communicating with the network. All that needs to happen with the product is for something to signal to it that it's corresponding address has been sufficiently funded. The extra cost I believe could be covered by money saved by requiring fewer in-store employees, and I wouldn't expect it to be too much different than current in-store security tags. Between salaries, benefits, and the tech, that's a lot of money you can reroute to something else.
I disagree vending machines are the same. Walking through the store and being able to pick up or look at the item is important. Online purchasing makes more sense for most people in many ways for many items, but people still go to the store to buy stuff. And need I mention the clothing industry, music industry, etc.?
I do agree that few (if any) stores could be currently successful in any location operating under a crypto-only business model. But this really is just one of many chicken-and-egg problems Bitcoin faces in general. It isn't unreasonable or too early to conceptualize something like this, in my opinion.