Passing CiviCRM invoice number to Moneris hosted payment page
If that is the case - Looking at the code I can see that in the confirmation page you are redirecting to Moneries Site for entering Card details and then once the payment is processed "Successfull/Failed" it is redirected again to CiviCRM Page. 

So at that time please read the order number from the payment details and update that against your contribution - which will be usefully for your client to link both the records - I guess this helps