First, note that I edited my last reply above to add a critical piece to the recommended Payment Variable Information URL field, which went missing somehow.
The email notifications are just to inform your partner when their product is sold. To calculate a revenue share, you can view your E-junkie Transaction Log for that product and the relevant time period, which will show you the number of orders and a revenue total for that product and timespan. Then just pay half that amount to your partner by whatever means you wish, or if you have a regular affiliate program, you could edit your monthly masspay.txt file to add a line for your partner before you upload the file to PayPal Mass Payment.
Using affiliate links to track revenue sharing for a partner/supplier would be complicated to set up and at least somewhat unreliable, especially if you have other affiliates. The affiliate referral is tracked for each order, rather than for each item in the order, and the last affiliate referral made for a given buyer is the one that gets tracked for their order. If the buyer used your partner's affiliate Direct Link to add that item to their cart, then at some point used another affiliate's link before taking their order to checkout, the latter affiliate would wind up earning all commissions on that order.
That said, if you really want to proceed with that approach, we explained it here before we realized how unreliable it could be: