Hi,

if you are looking to get the link to the current record, you can generate using the below code,

base_url = request.env['ir.config_parameter'].get_param('web.base.url')
base_url += '/web#id=%d&view_type=form&model=%s' % (self.id, self._name)

Using this code you can update your code,

def send_email(self):
subject = 'Commercial Invoice'
recipients = self.customer.email

base_url = request.env['ir.config_parameter'].get_param('web.base.url')
base_url += '/web#id=%d&view_type=form&model=%s' % (self.id, self._name)

message_body = base_url
template_obj = self.env['mail.mail']
template_data = {
'subject': subject,
'body_html': message_body,
'email_to': recipients
}
template_id = template_obj.create(template_data)
template_obj.send(template_id)
template_id.send()

The end user who access the email should have backend access to Odoo, such a way that he can see the view.

Else it will be nice to send the portal link.

base_url = request.env['ir.config_parameter'].get_param('web.base.url')
base_url += '/my/invoices/%s' % self.id

Thanks