Payzapp is a payment portal that does not support repayment of credit card dues. You can use it to clear all other bills like utilities, insurance, rent etc.
Smartbuy is a member loyalty program where you can buy gift cards, book your flight, hotel, train tickets etc.
You can use your HDFC Debit card to pay your other credit card dues on BillPay which is another payment portal attached directly to your HDFC bank account. There you can get 1% cashback of the total bill amount paid against your other credit cards.