hello.
To implement this feature you should to create hook on checkout/success controller and make some changes of your template.
1.
install our developer tools extension and do the clone of default template.
2. Install this template and set it as default template of your store.
3. Open file with hooks (usually it's a extensions/your_template_name/core/your_template_name_hooks.php file ) and paste hook-method for success page.
for example:
public function onControllerPagesCheckoutSuccess()
{
$that = $this->baseObject;
//if not final checkout page - skip
if((int)$that->session->data['order_id']){
return;
}
$that->view->assign('some_your_custom_variable', 'BLAH_BLAH_BLAH-TEXT');
}
4. Open file view/{your_template_name}/template/common/success.tpl and paste
echo $some_your_custom_variable;
OR
You can paste some static html directly into your own view/{your_template_name}/template/common/success.tpl file (without hooks data)