Hello Angelina,

Here's how to handle both of your requirements in Odoo 17:

1. Warning for Overdue Invoices When Creating New Orders

Option A: Built-in Credit Limit Feature (Simplest)

1. Go to Accounting → Configuration → Settings
2. Enable "Credit Limits" under the "Credit Management" section
3. Set "Default Overdue Limit" to 1 day (or your preferred threshold)
4. Now when creating a sales order for customers with overdue invoices, Odoo will show a warning

Option B: Automated Action (More Customizable)

1. Go to Settings → Technical → Automation → Automated Actions
2. Create new action:
   • Name: "Warn on overdue invoices"
   • Model: sale.order
   • Trigger: On Creation
3. In Python code section:

python

partner = record.partner_id.commercial_partner_id
overdue_invoices = env['account.move'].search([
    ('partner_id', '=', partner.id),
    ('payment_state', '=', 'not_paid'),
    ('invoice_date_due', '<', fields.Date.today())
])
if overdue_invoices:
    warning = {
        'title': 'Overdue Invoices Warning',
        'message': f'Customer has {len(overdue_invoices)} overdue invoice(s)'
    }
    action = {
        'type': 'ir.actions.client',
        'tag': 'display_notification',
        'params': warning
    }
    # Optional: Block order confirmation
    record.write({'state': 'draft'})
    return action

2. Automatic Follow-ups for Overdue Invoices

Using Odoo's Built-in Follow-up System

1. Go to Accounting → Customers → Follow-up
2. Click "Define Follow-up Levels"
3. Create levels like:
   • Level 1: 5 days overdue (Email reminder)
   • Level 2: 15 days overdue (Email + letter)
   • Level 3: 30 days overdue (Stronger wording)
4. Configure email templates for each level
5. Set up automated actions to process follow-ups daily

To Customize Follow-up Emails:

1. Go to Settings → Technical → Email → Templates
2. Duplicate the default follow-up template
3. Customize subject/body with:

   Dear ${object.partner_id.name},
   
   Our records show invoice ${object.name} for ${object.amount_total} is ${object.invoice_days_overdue} days overdue.
   
   Please arrange payment immediately.

Pro Tips

1. For the warning system, consider adding:
   • Overdue amount totals
   • Links to the overdue invoices
   • Option to override the warning
2. For follow-ups:
   • Set different sequences for different customer segments
   • Add PDF reports of overdue invoices to emails

Include payment links in reminders

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(Need further customization? Drop a comment—I’m happy to refine the solution!)

Hi,

By default, Odoo doesn't have the feature to show a warning when a customer has a payment due.

By simple customization, we can achieve this. 

1-  Inherit the sale order.

Python

from datetime import date

from odoo import api, fields, models



class SaleOrder(models.Model):
    """Inherits the model sale.order"""
    _inherit = 'sale.order'

    is_payment_due = fields.Boolean(string="Is payment due", copy=False,
                                    compute="_compute_is_customer_payment_due")

    @api.depends('partner_id')
    def _compute_is_customer_payment_due(self):
        """Function that check the validity of the KYC documents and
        Payment due for the customer"""
        payment_due = sum(self.env['account.move'].sudo().search(
            [('partner_id', '=', self.partner_id.id),
             ('payment_state', '!=', 'paid'),
             ('move_type', '=', 'out_invoice'),
             ('invoice_date_due', '<=', date.today())]).mapped(
            'amount_residual'))
        self.is_payment_due = payment_due > 0

XML

<record id="view_order_form" model="ir.ui.view">
    <field name="name">sale.order.view.form.inherit</field>
    <field name="model">sale.order</field>
    <field name="inherit_id" ref="sale.view_order_form"/>
    <field name="arch" type="xml">
        <xpath expr="//field[@name='partner_id']"
               position="before">
            <field name="is_payment_due" invisible="1"/>
        </xpath>
        <xpath expr="//sheet" position="before">
            <div role="alert" class="alert alert-danger" style="height:40px, width:30px, margin-bottom:1px;"
                 invisible="is_payment_due == False or state in ('sale', 'done','cancel')">This customer have payment due after the payment terms</div>
        </xpath>
    </field>
</record>

By using the above code, we can add a warning message while selecting a customer who has any payment due.

Eg:

If you want to send mail while creating the sale order, add the following function.

def action_confirm(self):
    res = super().action_confirm()
    if self.is_payment_due:
        email_values = {'subject': '#Add email subject',
                        'email_to': '#Add receiver email',
                        'auto_delete': False}
        mail_template = self.env.ref(
            'module_name.email_template_id')
        mail_template.with_context(company=self.env.user.company_id.name).send_mail(
            self.id, force_send=True, email_values=email_values)
        mail_template = self.env.ref(
            'module_name.email_template_id')
        mail_template.send_mail(self.id, force_send=True)
    return res

Mail template

<record id="mail_template_id" model="mail.template">
    <field name="name">Payment Due Alert</field>
    <field name="model_id" ref="module_name.model_sale_order"/>
    <field name="body_html" type="html">
        <div style="margin: 0px; padding: 0px;">
            <div style="margin: 0px; padding: 0px;">
                <p style="margin: 0px; padding: 0px; font-size: 13px;">
                    Hi,
                    <br/>
                    #mail contents
                    #
                    #
                    #
                    <br/>
                    Regards,
                    <br/>
                    <t t-out="object.company_id.name"/>
                </p>
            </div>
        </div>
    </field>
</record>

Hope it helps

1. Alert on Sales Order for Customers with Overdue Invoices

By default, Odoo doesn't show a warning out of the box when placing a Sales Order for a customer with overdue invoices, but you can achieve this by customization

2. Automatically Email Customers with Follow-up Letters for Overdue Invoices

This is supported out of the box in Odoo via the Accounting App > Follow-ups.

Steps:

1. Go to Accounting > Customers > Follow-up Reports
2. Configure Follow-up Levels (e.g., 1 day late, 15 days late, etc.)
3. Set:
   • Automatic emails
   • Follow-up letter templates
   • Add SMS if needed