What you describe has been a fairly popular request, so we're strongly considering eventually adding something along those lines as a new feature. From what we've discussed, you would be able to define for each product some affiliate, or just a PayPal email, that would earn a flat amount on each unit sold of that product, separately from and regardless of any other affiliate commissions. Those earnings along with affiliate commission earnings would be included together in the masspay.txt file you obtain monthly to pay your affiliates via PayPal Mass Payments.
Formal announcement of any new features going live would be posted to our Service Status and Updates forum here:
...and if you're into RSS, we have a feed of updates to that forum available for your RSS reader here:
Meanwhile, you can go to Seller Admin > View/Download Transaction Log, select a date range and specific product, then View Log to view just those sales. You can also Download Log from any log view to save that sale data as a spreadsheet file for additional calculation/manipulation on your own computer, if you wish.