Some day I'd like to enable the "Estimated Shipping" capabilities of productcart...however, one major lag in the pop-up window keeps me from doing so.  The form on which the customer provides basic geographical info doesn't perform any validation.  If a customer just tosses in their ZIP code (which I would do), the system will return an error that no shipping rates were returned.  If the user then goes back and puts in a state and a ZIP code...you get rates (applies to US orders only).

Please implement some basic form validation here...if a field is required...make it required on the page and don't accept the submit without it being completed.