Will said "Waitrose - and probably other supermarkets - have a hand scanner facilit. You pick your good off the shelves and scan them - totting up the price as you go.

What happens at the checkout if the scanner price is different from the till price?"

Well, as an experienced zapper user, you don't use the till. You pay at a dedicated till point where it takes the data from the zapper. If the bar code is in any way compromised you have to scan the item at the till separately or ask for assistance. So if they were to introduce varying prices it'd be the same process as for the tills - they'd do something with the bar codes and the zapper would reflect that. It would only be an issue if you were lingering in the store across whatever time was decreed to be off or on peak, but I would imagine you'd do all you could not to have that happen.

The Waitrose system allows for personal choices at reduced rates (you choose which products are on your own "reduced" list), so I'm sure it's able to cope with all sorts of changes.