It is a common request and on my list, but unfortunately it isn't a simple feature to add as whenever the shopping option changes, you need to generate a new signed cart as that has to happen server side because only the server has the encryption key. So that means some ajax query push when the shipping changes. Also, there will be no enforcement of regions --if someone selects "Priority USA only" they can go to Paypal and put in a shipping address of India. Google Wallet does this much better, as they let you push shipping options in the order, but sadly they are going away and Paypal you must either use their limited shipping options or do what you are asking for. But adding that feature will also make it easier to integrate with other payment / CC supplies as that is how shipping is usually done. If you want to help code that, please let me know.