Thanks - there's a write-up of the plugin available here:
http://www.affiliatewebdesigners.com...-dropshipping/
I actually think it should work with the Paypal 'Add To Cart' button - the reason I'd originally been hesitant about the 'Add to cart' button was if you have some products which are drop-shipped and some which are affiliate links - but if, for one entire country, ALL products are drop-shipped then I see no reason for the AddToCart button not to work with the existing plugin I made.