Now that I think about it, 15 cents per item is probably far too low to have much effect on people's behavior. Too many people wouldn't even notice or care. What would it take? 50 cents? A dollar? We still want to have an express line that is free for people with small purchases (in my opinion--maybe the store managers don't really care.)

Here's a complication: Sometimes there is nobody standing in the express line, so the clerk invites someone else to come through her line, even though they have more than 15 items. It's better than having her stand there idle. She would have to remember to switch off her surcharging temporarily.