WhatsApp Business API for overdue payment reminder

Use case overview

The scenario sends the customer a personalised WhatsApp template when their payment becomes overdue. The message includes the customer name, the outstanding amount, and the overdue date. A static URL button opens the online payment page.

Template example

Hi {{1}}, this is a friendly reminder that your payment of {{2}} is now overdue as of {{3}}. Please complete your payment at your earliest convenience to avoid service interruption. If you have already paid, please disregard this message.

[Pay]

The payment page URL button is fixed in the Meta template — only body variables are sent via the API.

Variables and purpose

• {{1}} — customer name
• {{2}} — outstanding payment amount
• {{3}} — overdue payment date

Filled-in example

Hi Alex, this is a friendly reminder that your payment of 150.00 USD is now overdue as of June 15, 2026. Please complete your payment at your earliest convenience to avoid service interruption. If you have already paid, please disregard this message.

[Pay]

When to use it

• billing and subscriptions
• b2b and services
• agencies and integrators

Business value

• Customer receives a WhatsApp reminder when their payment becomes overdue
• Message is personalised with customer name, amount due, and overdue date
• Static URL button lets the customer open the payment page directly
• Billing or CRM system tracks delivery status of the reminder
• Customer can complete payment without needing to reply in WhatsApp

Workflow

1. Billing or the accounting system detects that a payment is past due.
2. Customer phone number, outstanding amount, and overdue date are retrieved from the billing system.
3. A personalised template message is built with three body variables and a static URL button linking to the payment page.
4. The customer receives the WhatsApp reminder and can follow the link to complete payment.
5. Delivery result is logged for further collection or CRM follow-up.
6. Delivery progress is reported asynchronously — typically sent, then delivered (or failed/undelivered).
7. Your system receives status via webhook (hooks[]) or polls GET …/hookInfo?messageId=<id> and handles failures if needed.

Technical implementation

Prerequisites

• A 1MSG account with WhatsApp Business API connected and an approved message template.
• Customer phone number in international format (without + and spaces).
• Personalisation data: customer name, outstanding amount, overdue date.

Code examples

#!/usr/bin/env node

// === Configuration (replace "___" placeholders) ===

const API_BASE_URL = "https://api.1msg.io"; // production 1MSG API base URL
const CHANNEL_ID = "___";                   // channel ID from 1MSG dashboard
const API_TOKEN = "___";                    // channel JWT token (Bearer)

const TEMPLATE_NAME = "___";                // approved template name
const TEMPLATE_NAMESPACE = "___";           // template namespace (422 without it)
const TEMPLATE_LANGUAGE = "___";            // template language code, e.g. "en"



// === Test data ===
const TEST_PHONE = "___";            // client phone in international format
const TEST_CUSTOMERNAME = "___";    // {{1}} customer name
const TEST_PAYMENTAMOUNT = "___";    // {{2}} payment amount
const TEST_DUEDATE = "___";    // {{3}} due date

function normalizePhone(phone) {
  return String(phone).replace(/\D/g, "");
}

function assertConfigured(values) {
  for (const [key, value] of Object.entries(values)) {
    if (value === "___" || value === "" || value === undefined || value === null) {
      throw new Error(`Missing configuration value: ${key}`);
    }
  }
}

async function sendTemplateMessage({ phone, customerName, paymentAmount, dueDate }) {
  assertConfigured({
    CHANNEL_ID,
    API_TOKEN,
    TEMPLATE_NAME,
    TEMPLATE_NAMESPACE,
    TEMPLATE_LANGUAGE,
    phone,
    customerName,
    paymentAmount,
    dueDate,
  });

  const url = `${API_BASE_URL}/${CHANNEL_ID}/sendTemplate`;

  // params carries body ONLY. Button text is fixed in the Meta template — no button param.
  const requestBody = {
    phone: normalizePhone(phone),
    template: TEMPLATE_NAME,
    namespace: TEMPLATE_NAMESPACE,
    language: {
      policy: "deterministic",
      code: TEMPLATE_LANGUAGE,
    },
    params: [
      {
        type: "body",
        parameters: [
          { type: "text", text: String(customerName) }, // {{1}} customer name
          { type: "text", text: String(paymentAmount) }, // {{2}} payment amount
          { type: "text", text: String(dueDate) }, // {{3}} due date
        ],
      },
    ],
  };

  const res = await fetch(url, {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      Authorization: `Bearer ${API_TOKEN}`,
    },
    body: JSON.stringify(requestBody),
  });

  const raw = await res.text();
  let data;
  try {
    data = JSON.parse(raw);
  } catch {
    data = null;
  }

  if (!res.ok || !data || data.sent !== true) {
    console.error("Send failed. API response:");
    console.error(raw);
    process.exit(1);
  }

  console.log("Message sent to client.");
  console.log("API response:", raw);
  return data;
}

if (require.main === module) {
  sendTemplateMessage({
    phone: TEST_PHONE,
    customerName: TEST_CUSTOMERNAME,
    paymentAmount: TEST_PAYMENTAMOUNT,
    dueDate: TEST_DUEDATE,
  }).catch((err) => {
    console.error("Execution failed:", err.message);
    process.exit(1);
  });
}

module.exports = { sendTemplateMessage };

Immediate API response (synchronous)

• HTTP 2xx and JSON "sent": true mean 1MSG accepted the message for sending — not that it already reached the customer's phone.
• Save the `id` field from the response (value looks like wamid.…). Use it to correlate delivery callbacks or polling.
• The response may also include message and description — informational only.

Delivery status (asynchronous)

• Register a webhook (POST …/webhook) so 1MSG POSTs delivery updates to your HTTPS endpoint in a separate `hooks[]` payload (sent, delivered, read, or failed/undelivered when applicable).
• Optionally poll: GET {base}/{channel}/hookInfo?messageId=<id from sendTemplate>.
• In practice, delivery often completes within a few seconds — but that is not guaranteed by the API contract.

Common errors

• Invalid or non-normalized phone number
• Unapproved or missing template name / namespace
• No customer opt-in for WhatsApp business messages
• Template variable count mismatch (422 from API)
• Delivery failure — check status webhook and retry policy

FAQ

• Do I need an approved template? Yes — cold-start WhatsApp messages require a Meta-approved template.
• Can I customize the message text? Body variables are dynamic; fixed text and button labels are set in the Meta template.
• How do I check delivery? sent: true only confirms acceptance. Track delivery via webhook hooks[] or GET …/hookInfo?messageId=<id>.
• What if the message is not delivered? Log the failed/undelivered hook, verify opt-in and template status, then retry or fall back to another channel.
• Can I connect this to my CRM or backend? Yes — trigger the API call from your platform webhook or event handler.

Related

• Knowledge base
• Creating a template
• Sending a template
• SDK