If you have the offer and connect it to your card (unfortunately I don't have it), you can get up to 3 $10 credits when you spend $50 or more 3 times.
If you have the offer on multiple cards, don't connect it to one of the cards right away. Open up americanexpress.com in separate tabs and get the offer open on all of your cards, then click on the offer for all the cards. If you don't do this, the offer will disappear from your other cards one you click on it once.
You can load up on gift cards (to the grocery store or elsewhere) in $50 increments to stock up on 20% off.
From the Amex Details:
Where you can redeem your offer
For the purposes of this offer, a “supermarket” is a store whose primary business comes from the sale of a variety of foods such as meat, fresh produce, dairy, canned and packaged goods, but also sells a variety of household products, pharmacy products and pet supplies.
The following are NOT considered supermarkets:
- Specialty food stores such as a standalone cheese shop, butcher, or bakery
- Small corner grocery stores
- Gourmet shops
- Convenience stores
- Drug stores
- Natural food stores
- Large superstores including warehouse clubs, big box stores, supercenters, hypermarkets, and megastores whose primary business comes from the sale of items such as toys, electronics, furniture, clothing, sporting goods, household products, office supplies and beauty supplies, and which may also sell food in a space that is typically less than half the total store size