Amazon and the FTC haven’t publicly shared the subject line, probably to prevent scammers from copying it. The safest way to know if an email is legit is to check the sender domain (@amazon.com, or the official payment processor - i.e. PayPal or Venmo), avoid clicking suspicious links, and confirm through your Amazon account or the FTC refund page. Some keywords to search for would be Amazon refund, Amazon account, settlement, prime membership, etc. They started rolling out the payments in Novem