The Benefit Terms point to https://www.americanexpress.com/us/content/rewards-info/retail.html, which gives a vague definition of a US Supermaket along with a quite partial list of US supermarkets.
To earn additional rewards on supermarket purchases, the supermarket must be located in the U.S.
A supermarket offers a wide variety of food and household products such as meat, fresh produce, dairy, canned and packaged goods, household cleaners, pharmacy products and pet supplies.
(Superstores, convenience stores and warehouse clubs are NOT considered supermarkets.)
Examples of merchants* that accept the Card and where you can earn additional rewards include:
- Stop and Shop
- Whole Foods
- Online supermarkets such as FreshDirect
*This is not a complete list.
Examples of merchants where you will NOT earn additional rewards include:
- Specialty stores (e.g., fish markets, cheese shops, wine shops, and other specialty food stores )
- Superstores (e.g. Amazon, Target and Wal-Mart)
- Warehouse clubs (e.g. BJ's Club and Costco Wholesale)
I did try calling Amex (800-243-3888). They told me they don't have any list, and that I should instead ask each shop whether they accept AmEx and are regarded as US supermarket. I am hoping that some AmEx customers may have aggregated some list.