This seems like a bad idea. If I own a physical store and you’re in it I don’t want to encourage you to leave quickly. I want you in there as long as possible putting things in your cart.

wouldn't this encourage people to buy fewer items?

But customers are so used to confusing pricing labels you could just make the old (cheaper) price say: "Discount: $x".

Also if you hook up the change to motion detectors where you know no one is in the aisle, then you can skip it altogether.