Looking for good specific affiliate/referral plugin

So I only sell one type of product on my site that comes in packs. I want to star an affiliate program where for every 5 customers someone refers to me, they get a free pack. I want to be able to see how many customers a user has referred and I want to get notified when they reach 5 (also they can notified) so I can send them their free product. What extension/plugin will allow me to do this?

Thanks for the help!