Imagine walking down the aisle, normal day, no thoughts about the prices or any of that.
Then one day you walk down the aisle but this time you forgot your phone in the car.
Different prices. Then some one walks is coming close from the other end of the aisle. The price changes. They walk past, nonplussed. A few seconds later, it switches back.
The year is 2047. Individually tracked pricing algorithms determine prices for each customer. I am the local water man, who everyone pays a small fee to go buy clean water, because my high volume of purchases means I get a slight discount. In only 34 more years I can pay off my 8th grade education and start thinking about a down payment on a double sleeping pod.