The store is only going to pay out a referral commission to one cash back website, so only one website is going to pay you for any given purchase. The trick is to stack different types of rewards, e.g. Portal cash back + store rewards (e.g. CVS extrabucka, Sears ShopYourWay) + third party rewards like Plenti (where applicable) + credit card points + coupons and/or mail in rebates. And if possible, going through a portal to buy a gift card and then going through a portal again to make the actual purchase.