How do I attach an invoice PDF to an email in the Dukapress plugin for WordPress?

function dpsc_pnj_send_mail($to, $from, $name, $subject, $msg, $attachment = FALSE) {
    global $wpdb;
    $headers="MIME-Version: 1.0" . "\r\n";
    $headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
    $headers .= 'From: ' . $name . ' ' . $from . '' . "\r\n";   
    if ($attachment) {
        if ($dp_shopping_cart_settings['dp_shop_pdf_generation'] === 'checked') {
            $mail_attachment = array(DP_PLUGIN_DIR. '/pdf/invoice_' . $attachment . '.pdf');
            @wp_mail($to, $subject, $msg, $headers,$mail_attachment);
        }
        else {
            @wp_mail($to, $subject, $msg, $headers);
        }
    }
    else {
        @wp_mail($to, $subject, $msg, $headers);
    }
}

The above is the code we’re using on another site. Works.