如何在dreamveation的v4.0快速结账中添加额外的评论字段？

Question

I have an opencart website, recently i developed an android app for it and for the checkout i used quick checkout v4.0 by dreamveation.com. but the problem is in the app's checkout the comment field is shown but on pressing the confirm button the comment is not getting updated to the database.

this is the tpl file

Please select your prefered slot of delivery.
<div class="select-input form-group">
<select name="comment" class="form-control comment-select" data-refresh="5">
            <option  value="slot1" id="slot1" selected="selected" >Slot 1 : 9:30 am - 12:30 pm</option>
            <option  value="slot2" id="slot2" >Slot 2 : 1:30 pm - 5:30 pm</option>
            <option  value="slot3" id="slot3" >Slot 3 : 6:00 pm - 9:00 pm</option>
</select>

and this is how im trying to get it in the d_quickcheckout.php

$this->session->data['comment'] = $this->request->post['comment'];
        $data['comment'] = $this->session->data['comment'];

but it gives me an error saying that there is nothing with index confirm..