For those wondering...
The bank’s platform requires a POST request to this url: https://www.deltapay.gr/entry.asp
Now the data that it needs to request a payment is the following:
MerchantCode provided by DeltaPAY administrators. - This should be set by the admin and saved
The amount you want to charge (decimals should be formatted with a comma separator e.g. 10,50) - This should be obviously generated by magento depending on the order.
ISO standards 4217 numeric currency code (e.g. 978 for Euro or 840 for US Dollar) - There should be a dropdown with some basic currencies, or just Euros should be ok.
The client’s name as it appears on the card - User input in the end of the order
The client’s email for verification purposes - User input in the end of the order
The number of installments for this payment. - Set by the admin from a text field
Seems easy enough at a quick glance. Those are your requirements, and they are plainly stated. Design a script that includes a form with those fields (show in bold above) set to the criteria described, set method to post and action set to the URL they provide.
Unfortunately, it's not that easy.
We cannot likely help you here because some of the elements of the form is relevant only to an insider who knows the specifics of that bank, i.e. any developer who attempts this would be working with the bank to create the proper form. There are legal and security issues involved not to mention this seems like a paid consulting job anyway - not just freebie code advice like we normally offer here.
And on top of all that, the developer would have to have specific experience with the Magento platform because that's part of the requirements as well. You might get a response here as to that, but ultimately posting on the Magento site as you did was the proper and best action, I think. As for us, on this forum, we likely can't help you beyond basic HTML form design which I suspect you already know as you noted you were working with XML on the other site. If you start writing code and get stuck, sure, that's more what we can do for you here.
Good luck and hope you find someone.