Everyone that used HDFC DC on Unipay should follow this process to get the quickest resolution for a refund.
1) Login to HDFC netbanking site, Scroll down to the last and click on the + sign that u will notice under quick links section, after which you will see a Need Help option > Contact Us > Online Dispute Resolution, you can login and follow next steps with some details autofilled and rest you have to fill yourself.
Or
2) Follow the below link and provide all the transaction details.
https://leads.hdfcbank.com/applications/webforms/apply/HDFC_CustomerCenter/Customer_Center.aspx
They will acknowledge and revert back in a day and your refund will be issued in
max 4 days or even less even if the resolution time given is very long. If you have no transaction ID or anything, dont have to really provide anything.
Just check bank statement for merchant name and date of failed transaction.