Affiliate commission on marketpress: per product line.

hi guys,
i have affiliates installed, and marketpress, and the doodaah that adds them together.

So i would like to pay out x amount to referrers who send someone off to go buy an event from me.

but - we have 4 different products, and i'm only giving commission on 1 of the 4.

(the other are add-ons, think of a phone, with accessories- i only give commission on the phone)

how do i do that?

thanks!