Just got an email from AMEX, as I setup Apple Pay with that (Costco) card, and thought it would be of interest here:
"A few things to note about Apple Pay:
When you turn over your Card in Apple Pay, you will see two different sets of numbers. One will be the last 5 digits of your American Express Card number. The other is the Device Account Number, which is a unique number assigned to this Card on your Phone.
If a merchant asks for the last 5 digits of your Card after youve made a purchase with your American Express Card in Apple Pay, please use the last 5 digits of the Device Account Number."