WOW not sure I'd want to shop on that site if my price changes that often. ;)

At any rate you could create a webpart that would get the current users cart information and on page load or init update the cart items. Creating a webpart will allow you to put this "update" feature on any page you have cart information. I'd believe placement on the page would be important as well. You'd want to have it load before the cart information. This way you won't have to add any custom handlers in code and/or hard code a page URL.