I know this sounds evil, but for the moment, the best way to prevent screen scraping may be to use Flash. Flash now supports forms, submissions, authentication, yada yada yada.

This comment was not intended to address usability, nor the ability of non-Flash enabled browsers to purchase your product. But Flash is becoming pervasive, and is available on the platform that is most likely to be your customers browser of choice.

This will change, no doubt, as clients side Flash tools become as pervasive as various CPAN modules like LWP, etc. But for now, it seems to be the least likely method to address the issue you're concerned about.

--Chris