Hello Lazark,

You can use message_post() method to show notifications when user change quantity to 0 in write method. In message_post method you can add partners who are going to notified, you can add your message as well.

e.g.

record.message_post(body=body, partner_ids= record.partner_id.ids)

Hope it will help you.

Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari