Wrong credit card EMI charges by Amex card

I had purchased a product from amazon and used my Amex card for paying the money
worth 18k(6 EMIs of 3k). However I didn’t like the product and returned it for which
Amazon had refunded the same amount to my AMEX account but unfortunately AMEX
started charging EMI monthly without checking the refund they had received from amazon
and I never used to check my credit card as I had trust on the company brand name.
Today I had called up the customer care helpline for this mishap but they said they
cannot do anything about it and advised me to foreclose the (not taken) loan and they
cant work on refund.

DEBOTTAM
KOLKATA 700010, West Bengal
Email: dxxxxxxxxxxg@gmail.com