hi Mahamed,
in this case I want it to run without returning it, since it's an automated processing of invoices. Is this possible?

Hey again,
Yes, It is possible but in a different way. In this cas you can't use the function action_invoice_sent because this function will not send the invoice, it will open the Wizard "account.invoice.send" then this wizard has it own function the send the invoice (_send_email). I'll update my response according to your needs.

it makes sense. In that case, how can I send the invoice?

Dear Joao,
I've updated my answer.
Good luck.

wow, that is so great! looked all over the internet to find something like that. Than you so much.

One doubt: what is the "self.ids" in this line?
lang = template._render_lang(self.ids)[self.id]

it's almost working. The invoice is marked as sent, but the email is not actually sent (it's not even on emails list that failed on odoo settings, so it's not an SMTP configuration problem)
I tried to look into the system's 'account.move.send' model and didn't find what could be the possible cause.

found the solution, 3 methods must be called before send:

wizard = self.env['account.invoice.send'].with_context(ctx).create(values)

wizard._compute_composition_mode()
wizard.onchange_template_id()
wizard.onchange_is_email()

# If you want to send without printing the invoice use this function
wizard._send_email()

now, the only thing that is not working is the invoice 'move_to_sent' is not set as true.

Dear Joao,
for this line: lang = template._render_lang(self.ids)[self.id],
I did a mistake and i've updated the code.
_render_lang: Given some record ids, return the lang for each record based on lang field of template or through specific context-based key.
This line is added to ensure that the email will be sent in the correct language.

Good catch Joao!

found the solution, 3 methods must be called before send:

wizard = self.env['account.invoice.send'].with_context(ctx).create(values)

wizard._compute_composition_mode()
wizard.onchange_template_id()
wizard.onchange_is_email()

Dear Joao, could you please add a positif vote on my solution. :)

added a positive vote :)

also could you change your post to reflect the corrections we discussed here? Just to help someone in the future that stumbles upon this post.

Also, in this line 'model_description=self.with_context(lang=lang).type_name,' self is actually invoice_object? At least I had to use it

Also, I had to create a composer instance:
composer = http.request.env['mail.compose.message'].create({
'composition_mode': 'comment'
})

then use it in the values:
values = {
'model': 'account.move',
'is_email': True,
'res_id': invoice_object.id,
'template_id': template and template.id or False,
'composition_mode': 'comment',
'composer_id': composer.id,
'invoice_ids': invoice_object.ids,
}

Thank you for the positive vote :)
Yes, you were right, it should be invoice_object.
Updates are made on my response.
Thank you for your feedback, it was like a teamwork.

Hi Mohamed

thank you so much for your help :) yes it felt like team work. With this, now anyone can implement the same

Hi Mohammad,
The above code fits to my requirements, but the only problem is it is generating 2 emails.
1. from user to customer
2.from user to user itself (which is not required)
Can you please help me ?
I am using odoo 16 enterprise. Thanks in advance