We currently don't have any built-in way to accomplish what you describe automatically. Development does have a wishlist item requesting that we should allow a specific affiliate to earn a flat commission amount on every sale of a given product, which you could then pay out monthly along with your regular affiliate-referral commissions, so that feature may be coming eventually.
For now, the best we can suggest would be viewing your Transaction Log to calculate and pay out shares manually. You can use the Product menu in the Log and click View Log to see just that product's sales to calculate, or you could just download the whole log for a given date range and open that in your spreadsheet program, where you can sort by Item Number and calculate sales that way.
