Once you enter the store and set your Quantity and Products Types then you have to enter your shipping address and Billing Address and then after these three page movements or when you are in the process of going thru them you decide that your Order Qty or Product Items Need Updates or changes.
You Need to return to Order Summary for an UpDate...You are Now in trouble because You have to Go back To the Shopping Cart/Order Summary to update the Product List and or Qty.
Then You have to re-Enter Your Shipping & Billing Addresses again and go thru these Multi. steps again.
Please Remember.. that most users are new and are also first time users of an e-Commerce store .
Therfore Site Navingation, and the ability to Update and Rembering what was previously entered is important to a Novice User.
Most e-commerce stores allow Updates all the way up to Summit the order to the payment Processor.
And the miminize the data entry for Customer Billing and Shipments entries.
Also once a user gets to the CheckoutShippingMethod.ascx and Notice that the Cost is to High due to the order Qty and or the added Shipping Cost is to high and Needs to be alter the Order how does he/she return to the shopping cart without going thru all the steps or enter the shipping infor and billing info again. Thier are many reasons a buyer will need to know and have a way to modify the order all the way and upto the commit stage.