That's more a PayPal question than a Django one. But at Mozilla we use adapative payments with Django. Our codebase is rather big so might be hard to follow, but the key PayPal stuff is here:
https://github.com/jbalogh/zamboni/blob/master/apps/paypal/__init__.py
It doesn't answer your question, but gives you something to look at ;)
Cheers.
--
Andy McKay
an...@clearwind.ca
twitter: @andymckay