Final answer:
The best way to issue a refund for returned merchandise purchased with an American Express card is to credit the customer's American Express card for the amount of the refund.
Step-by-step explanation:
When a customer requests a refund for returned merchandise that was purchased with an American Express card, the best way to issue the refund is to credit the customer's American Express card for the amount of the refund. This can be done by processing a credit transaction through the merchant's payment processing system, using the same method as the original purchase.