when i used my voucher, i simply call the TAJ hotel, they asked me to provide the voucher code on the email they sent me for hotel confirmation. also if the charges are extra mention that you will pay the the property the remaining amount, they didnt ask me if i have the card or not you can pay the remining amount with any card or cash or upi. make sure when they ask you to provide the voucher no. and code. provide them the full email screenshot. so its clear that you received the voucher from amex on your email id. (To, From - email ids).