[jchw]

Well, in case of the server-side shopping cart, the server would have enough state to know if the price has changed since an item was added to the shopping cart, so the client wouldn't necessarily need to participate in preventing this scenario. I'm sure in practice, the checkout confirmation process for modern e-commerce sites also takes extra precautions to ensure that it can only happen exactly once and that it fails if any details change during the process of checking out.