In order to make your payment at Papaki via web-banking, use the Electronic Payment Code, which you find in the ordering email (or in the Control Panel if it is a service renewal / upgrading).
Specifically, the procedure you should follow is:
1. Login to Ethniki's web-banking.
2. In the Management Environment of your account, select the Payments tab, followed by "Companies & Organizations" and "Other Companies." On the page that appears, fill in the billing account, the Electronic Payment Code, and the payment amount.