In a spreadsheet program like Excel, iWork Numbers, or (free) Gnumeric, you can Select All and then sort rows by the Affiliate ID column (so each affiliate's sales will be grouped in consecutive rows), then just select all the cells in the Affiliate Share column for a given affiliate, and the spreadsheet should show an instant sum of those cells somewhere, such as down in the status bar at the bottom.
For a different approach, you could just search your PayPal account history for all payments you've sent to each affiliate's PayPal Email in the past year, and sum up those amounts to see if the affiliate received more than $600 from you in that year.