American Express has a card "Blue Cash Preferred". You get 6% back on Groceries (up to $6k/yr), 3% back on gas, and the card has a $75 annual fee.
If you only use the card to spend $100/week on Groceries, you get $312 in cashback rewards. Subtract the $75/yr fee, and Amex paid you $237 to use their card that year.
(Amex isn't the only card like this -- there's lots from MasterCard and Visa as well. This card in particular just happens to be one of the rewards cards that I know the details of.)
If you only use the card to spend $100/week on Groceries, you get $312 in cashback rewards. Subtract the $75/yr fee, and Amex paid you $237 to use their card that year.
(Amex isn't the only card like this -- there's lots from MasterCard and Visa as well. This card in particular just happens to be one of the rewards cards that I know the details of.)