UPI is the most feasible option for Amex Credit Card Payment now

From this month onwards both Mobikwik and Amazon stopped supporting Amex CC bill payment and Cred switched to UPI , cred is making the payment based on the details entered and will convert it to a UPI payment instead hence you will only find the bank's UPI as the payment methods and other UPI app's aren't available

UPI format for Amex Payment - AEBC[Card Num]@SC

sample format - AEBC371234567891234@SC

if you directly do it via send money option in amazon pay then Amazon wallet money balance can be used as it will be treated as a UPI transfer

Error from Amazon