Paypal has stated that they prefer adaptive payment style, where when a sale is made, then the split is made from that sale.
The alternative to that is RAP, where first sale goes to product owner, second sale goes to affiliate and then it switches back and forth. the downside is that if you make 1 sale, you get 0% (when it should be 50%). If you make 3 sales, you get 33%.
But anyway, here is a link:
https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_APIntro