any application/platform which allows unbilled/custom amount HDFC bbps credit card payment

You can pay custom amount via sbi unipay. Usually their server have some issues so try with low amount payment. Pay with millennia/platinum You will get cashback on your hdfc Debit card as well.

Sadly, this isn't possible now..ever since HDFC joined BBPS.

You can only pay amount less than or equal to due amount now..

No more Pre-payments

Checked : Paytm, SBI Unipay, Jio Finance etc..

So Basically, 1% CB for HDFC CC is no longer now