need help with editing a file for a contributed module

[Xbox Memberships]

ok not sure if this is in the right place in the forums so please feel free to move it mods if in wrong place :s

i have installed the max mind reduce card fraud module thatis in the downloads section this involves overwriting the order.php in includes/classes

only problem is I have a module installed which would be rendered useless if i was to overwrite this which was custom built so i need to add the following lines to it....

//start coderXO mod
$xboxcodes = array();
$xboxcodehtml = "";
if($this->products[$i]['model'] != "" && $this->info['order_status'] != DEFAULT_ORDERS_STATUS_ID)
{

$xboxcodes = getXboxcode($this->products[$i]['model'],$this->products[$i]['qty'],$zf_insert_id);
if(!empty($xboxcodes))
foreach($xboxcodes as $xcode)
$xboxcodehtml .= "\r\n[Xbox Code: ".$xcode. "]";


}
// end coderXO mod


// build output for email notification
$this->products_ordered .= $this->products[$i]['qty'] . ' x ' . $this->products[$i]['name'] . ($this->products[$i]['model'] != '' ? ' (' . $this->products[$i]['model'] . ') ' : '') . ' = ' .
$currencies->display_price($this->products[$i]['final_price'], $this->products[$i]['tax'], $this->products[$i]['qty']) .
//start coderXO mod
$xboxcodehtml.
// end coderXO mod

where exactly would i put this?? I have asked in the relevant thread but as the thread was last commented on 2 years ago i hold no hope of a reply there

also i have the following modules that may also be affected but not sure....
better together with admin panel for it
reward points module

I am runing it on zc 1.3.8a

I have attached the order.php within the zip file itself for reference

wouldn't normally ask like this but i cannot resume trading until this is resolved as the fraud risks in my buisness are too much

[Xbox Memberships]

just tried it myself and boom! grey screen instead of checkout step 2 of 3