Every site I've seen that uses the PayPal shopping cart has separate buttons for international and local sales.
Basically you can't vary shipping by region unless you have a separate inventory item for each shipping option, and as far as I know you can't combine postage (unless you make the shipping options a separate item to add to cart).
There are clunky ways you can have all these options a bit more streamlined, but you're better off installing a real shopping cart (free or otherwise) and just using PayPal to process the payments.
A forum community dedicated to T-shirt merchants, professionals and enthusiasts. Come join the discussion about graphics, commerce, dyes, prints, collections, guides, displays, styles, scales, equipment, accessories, reviews, classifieds, and more!