The thirteenth item, and any subsequent items brought to the express checkout lane, will not be sold to you. It doesn't matter what that item is, or how important it is to you, relative to the first twelve items scanned. The machine will not accept it. The grocer (who, when I rule the world, shall be a robot) will politely inform you that he or she is very sorry, but this is the "12 items or fewer" isle, and they cannot sell you any more than that. You'll have to go get in line in one of those other isles.
Despite its apparent charm, and its being programmed to speak with a British accent (current grocery store scanning robots, with their obnoxious Star Trek computer voice, will be no more), the robot will not change its mind based on the fact that you are a charming elderly woman, who in no way will remind it of its robot grandmother (who I guess might be very much like the Star Trek computer-voiced checkout machines of today).
You will then be sent on your way, perhaps with a pamphlet explaining the importance of the grocery store maintaining its credibility with regards to the express checkout isle, and that had the robot sold you the 13th through 25th items as you had wanted it to, this would have been horribly unfair to the people in line behind you, who were adhering to the store's policies regarding the express checkout lane.
Won't that be nice?