I don’t recall where - maybe in the shopping thread here or some blog post comments - but I have seen multiple people confirm that they received the Amex offer cashback when they used the shopping portal instead of the Amex link. I suspect they/MF want to reduce the double dipping, but don’t actually have a way to prevent it based on the integration with Amex (which presumably just looks for MF transactions coming through). Also, I have received two MF cashback credits from Citi Merchant Offers when going through the AA portal.