Is there any difference between Lifetime Commissions and setting cookie lifetime to unlimited time ?
Lifetime Commissions is more secure as defining unlimited cookie lifetime, because it will work even e.g. if customer will buy new computer or delete all types of cookies from his computer. Functionality of this plugin is based on fact, that you can identify customer by any unique identification (e.g. by email). After customer will make first sale, Post Affiliate Pro will make relation between customer and affiliate and all next sales from this customer will be assigned to affiliate even if customer will buy next products on another computer. Even if customer will be refered by any other affiliate on another computer, still will be assigned commission to first affiliate, because Lifetime Commissions have higher priority as any other type of tracking (e.g.