Green bags are a great idea. My mum forgets to bring them with her to the shops.
There are two problems that I can see with green bags: one, people use them, leave them in their car and forget to bring them with them to the shops, and two, the green bags don't get as far as the car, they're in the
cupboard under the sink or the storage equivalent.
Therefore I propose a little robot that is designed to make sure that people put their green bags back in their car, and to take them out with them into the shops.
The robot would come with little sticker-chips that you attach to the inside of your green bags, and they would provide the robot with information as to their whereabouts, and another, main one in your car. The Robot would have a speaker attachment to the car, so that a recorded message, overriding the radio, cd player, etc, saying 'remember your green bags' will call out as you enter the car park as it'll tell you as it's got GPS on it, with special attention to supermarket, food and alcohol shops.
When you get home, unpack your bags, what you won't notice is that the little critter has gotten out of its solar charger on your roof, attached itself to a green bag, and will proceed to irritate you by loud 'put the bags back in the car' messages, and generally getting underfoot.
Once the bags are back in the car, the little Reminder Bot will get the owner to put it in its dock on the roof of the car, or it would just switch off until the next time.
The bot would be about the same size as a large coffee cup, with wheels. And no 'off' switch.