@Chris Coyier - Hmmm, I wonder - do you think some sort of "reverse affiliate" system might actually work better?

Something along these lines: the website owner lists your digital product for sale and, if one of their users goes ahead and makes a purchase, the website owner / affiliate automatically pays you the wholesale price (say, maybe, 66% of the retail price, which would be $17.82 in your case) from their own PayPal account, but they get $27 from the buyer.

If the purchase is legit, they've just made a profit of $9.18; if it turns out to be fraudulent they gain nothing although you could refund them the wholesale $17.82 (minus transaction fees) if you feel that they were not part of the fraud i.e. if the majority of their sales have been legit.

At no point in that chain would there be any incentive for the affiliate to engage in fraud because you're not actually paying them a commission, they would simply be reselling at a profit, like most other businesses in the world. If a stolen credit card is used to buy a pair of Levis in Macy's, Levis doesn't take the hit.