This really depends on your affiliate tracking system and not the search engine. The way you would set something like this up is basically assign a new affiliate code for each search engine you want to track and for every hit that arrives to your page, you would need to embed the keyword used to arrive there into a hidden parameter and carry that parameter from page to page until the credit card transaction is complete. It's very useful information to track which keywords are making sales and which are a waste of money, but it's not a system that is very easily setup.
Alternatively, you can use hidden 1x1 images that have the keyword embedded in the request url (ex: spacer.gif?key=keyword1+keyword2) and then you can track the keywords this way from page to page.
Just my
WG