Order Total

I have an web application that allows a user to order items online. The user selects an item from a list on the order items page and enters the quantity. The quantity and unit price of the item is calculated on the order summary page. And than on the process order page, the calculated total from the order summary page is represented as the extended total for that item.

My problem is when that same user in a single user session orders another item, the extended total for the second item is not being added to the extended total of the first item. In other words, I would like to know the code to keep a running total for this user. I keep getting only the extended total for the current item exclusive of the previous item in that user session.