Drupal commerce - what page/function should i call after an off site payment
i used those two

// Return to the previous page when payment is canceled
'cancel_return' =&gt; url('checkout/' . $order-&gt;order_id . '/payment/back/' . $order-&gt;data['payment_redirect_key'], array('absolute' =&gt; TRUE)),

// Return to the payment redirect page for processing successful payments
'return' =&gt; url('checkout/' . $order-&gt;order_id . '/payment/return/' . $order-&gt;data['payment_redirect_key'], array('absolute' =&gt; TRUE)),


but also have an IPN. (your NOTIFICATION_URL)

don't forget to create a commerce transaction with the feedback from the carrier !

maybe have a look at another module, like postfinance (by me),
or paypal (by Ryan - the commerce god ;))