Calculate Shipping Button

I currently have shipping configured for my osCommerce site [login to view URL] I would like a button to appear whenever there is something in the cart which will use the same shipping calculations that are used for checkout. Currently, they must go through the registration and cart to see shipping charges. I would like for users to be able to decide if shipping is affordable before going though cart steps. They are now calling us on the phone before they enter the cart. Note - this must use the same shipping calculation so that the values match the final checkout shipping charges.