Online purchases or purchases made at chain stores are not eligible for this offer. You can shop a any small business that accepts Amex. It can be a fruit store, the pizza shop, toy store etc. I heard that some stores will be open on Motzei Shabbos because of this. Mostly Music sent out an email to everyone on their mailing list that they will be open.
You have to register your card before taking advantage of the offer.
https://sync.americanexpress.com/sbs2011