Order details on the check out success page

I've seen another thread asking the same question but it was old, locked and didn't have any positive answer.

Basically, I've installed the module Optimal Payments but the bank wants us to have specific informations on the last page of the transaction, including most of the order details. There doesn't seem to be any ready-made template on the Add-On section, which surprise me a little.

I wanted to make sure before I start, as anyone ever done this before and could provide some help?

The file to edit would be:

includes/languages/english/html_includes/your_template/define_checkout_success.php

I know which files to edit (tpl_checkout_success_default.php will have big changes also) but the problem is the actual code, which won't be short because of all the data that needs to be accessed. That's why I wanted to know if there was code out there I could use.