I don't use the button code myself, but my affiliates use it - isn't that enough? See this post, where the owner sells my product: http://www.bloggdesigns.no/?p=411
If you click these affiliate links and buy the book, the cookie should remember where the sale comes from, right? Paypal would report the sale through the return of the IPN to your ipn url, I believed.
If not, is there a way to use your button code and get straight to paypal without (visibly) passing by e-junkie?