Cashback can be stacked with sitewide sales and discounts offered by the stores. 

We advertise a number of eligible coupon codes and offers on our 
Coupons page and Store pages.  If a coupon code is not advertised on Cashrewards, the use of it may result in your purchase not being eligible for cashback. 

Cashback cannot be stacked with other cashback sites as only one cashback site can receive commission on a purchase.