The following is a quick and dirty way to send an email to the user informing them of an order status change. This is a kernel hack of the kernel shop module view orderlist. This solution was posted to the forums and then cleaned up here: http://share.ez.no/forums/setup-design/webshop-how-do-i-trigger-a-function-when-changing-status#comment84390

kernel-shop-orderlist.php

<?php
// begin snippet

if ( $http->hasPostVariable( 'SaveOrderStatusButton' ) )
{
    if ( $http->hasPostVariable( 'StatusList' ) )
    {
        foreach ( $http->postVariable( 'StatusList' ) as $orderID => $statusID )
        {
            $order = eZOrder::fetch( $orderID );
            $access = $order->canModifyStatus( $statusID );
            if ( $access and $order->attribute( 'status_id' ) != $statusID )
            {
                $order->modifyStatus( $statusID );

                // STATUS EMAIL ....
                $email = $order->attribute( 'account_email' );
                $tpl->setVariable( 'order', $order );

                $templateResult = $tpl->fetch( 'design:shop/statusemail.tpl' ); // create a template
                $subject = $tpl->variable( 'subject' );

                $mail = new ezcMailComposer();

                $emailSender = $ini->variable( 'MailSettings', 'EmailSender' );

                if ( !$emailSender )
                {
                    $emailSender = $ini->variable( "MailSettings", "ShopAdminEmail" );
                }

                $mail->charset = 'utf-8';
                $mail->addTo( new ezcMailAddress( $email ) );
                $mail->from = new ezcMailAddress( $emailSender );
                $mail->subject = $subject;
                $mail->htmlText = $templateResult ;
                $mail->build();

                $transport = new ezcMailMtaTransport();
                $transport->send( $mail );
            }
        }
    }
}

// end snippet
?>