Using external online payment systems with minivend?

[Jarno Niemela <jargon@iki.fi>]

I'm implementing a web shop that uses online
bank transactions and external payment
systems (locals don't like using credit cards...)

The payment system works in a following way:

-When the customer has clicked the order button,
he gets taken to a new page, where he's shown
a from which takes to the banks online payment
page

<FORM METHOD="POST"
ACTION="https://verkkomaksu.leonia.fi/vema.nd/Verkkomaksu/pgTunnistus">
etc...

-When the user has completed the transaction with the bank, the bank
returns the user to a page
specified by the shopowner (ie. accept transaction
page)

-And the shop is supposed to email the order data
to the shopkeeper only after the payment
is accepted

Diagram:

----
checkout
----
 |
 |
----
Form to the bank (still in minivend)
----
 |
 |
----
Bank  (Outside minivend)
----
 |
 |
----
Payment accepted 'page'
----
 |
 |
----
receipt to customer and email order to shopkeeper
----

So what I need to do is to add one page between
the /ord/checkout.html (the order page) and the
system sending the email to the shopkeeper
and printing the receipt. 

The add on should support the order form and 
at least two result pages (ok and fail).

The question is: 
-What would be the best place to modify the 
minivend to accept external payments?

-Do you already have an interface for such
operations?

-Which file should i modify? 

Jarno Niemela