I strongly suspect that City Merchant Offers and Simply Miles are powered by the same provider. If you look, the merchants listed on both sites are basically identical and the website layout is also very similar. The only difference is the Simply Miles give you miles per dollar spent or a lump sum of miles for meeting certain thresholds, while City Merchant Offers are cash back. If you look at Motley Fool, I believe the T&C says that it's not stackable on both Simply Miles and Merchant Offers. If it's the same provider, I think they'd be pretty reliable at weeding those out. Other deals, like Uber, do not have the no stacking warning, so I think you can get cash back and miles. As for stacking Simply Miles/Merchant Offers with eShopping, perhaps that will work with some merchants. For a merchant like Motley Fool, I assume they'd have to identify stacking on the back end since it's doubtful eShopping (Cartera) talks to Citi Merchant Offers and Simply Miles. Presumably when Motley Fool pays out, they get identifying information that would show someone who double-dipped.