Express checkout before shipping calculation

This is a general question – but it refers particularly to YITH plugin that I’m testing on my new project: I wonder how could I use the express ‘pay now’ option properly on checkout page before customer fills all mandatory billing (o shipping) fields. In case I’ve got different shipping costs for different countries/regions/zip codes or whatever, how can the customer pay the actual total by clicking on ‘pay now’ buttons avoiding the form?
This is something I can’t underdstand.

By the way, the classic checkout processing with Paypal/ Credit cards as final option in shipping methods after filling the form and calculating the shipping fees works perfectly.