Found it in my "AmEx offers and benefits" online account. YMMV
Get a one-time $40 statement credit by using your enrolled Card to spend a minimum of $50+ in one or more transactions online at marthastewartwine.com by 9/30/2017.
Note, offer availability will vary by cardholder. Be sure to login to your American Express account to see if you're eligible/qualified to activate the offer on your card.