with express checkout the workflow of the site will be:
Go to the paypal server for the customer to enter the Billing and shipping details and set the transaction.
Paypal redirects back to your site, to the pp_confirm page
on the pp_confirm page, the shipping price will be displayed with the option for the customer to either submit the order to be processed or cancel the order.
all the billing and shipping details are entered by the customer on the paypal server. The shipping cannot be calculated till paypal returns the customer to the confirm page on your site.