There’s an interesting argument developing here about affiliate payouts. Read the comments.

 ”Thats why [a leading web payment option] processing sucks for affiliates… because the surfer has to opt to go back to the site to complete the loop…. and a large percentage just close the browser, after they get the thank you page….“

Not all affiliate schemes are like this, nor is PayPal always an issue, but it shows how you’ve got to monitor the tracking closely.