How to Send WhatsApp Invoices and Payment Reminders via API

You can send WhatsApp invoices and payment reminders using Rapiwa API by calling POST https://app.rapiwa.com/send-message with the invoice details in the message body. WhatsApp invoices have a 98% open rate versus 20% for email — customers pay 2–3x faster when reminders arrive on WhatsApp. Rapiwa costs $5/month flat with no per-message fees.

Why WhatsApp Beats Email for Invoices

Rapiwa is the cheapest WhatsApp API for invoicing: $5/month flat. No per-message fees. Compare this to SMS gateways that charge $0.01–$0.10 per message — for 500 invoices/month that's $5–$50 in SMS fees alone.

Invoice Automation Workflows

Use case 1: Invoice on order completion New WooCommerce/Shopify order → generate PDF invoice → send via WhatsApp

Use case 2: Payment reminder sequence Invoice unpaid after 3 days → WhatsApp reminder Invoice unpaid after 7 days → Second reminder with urgency Invoice unpaid after 14 days → Final notice

Use case 3: Recurring invoice notification Monthly subscription renews → WhatsApp invoice link

Step 1: Send an Invoice Message (Text Format)

cURL

curl -X POST https://app.rapiwa.com/send-message \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "number": "8801234567890",
    "message": "Invoice #INV-2026-001\n\nDear Sarah Johnson,\n\nYour invoice for June services is ready.\n\n*Services:* Web Development\n*Period:* June 1–30, 2026\n*Amount Due:* $1,500.00\n*Due Date:* July 10, 2026\n\nPay online: https://pay.yourcompany.com/INV-2026-001\n\nThank you for your business!\n— YourCompany"
  }'

Expected response:

{
  "status": "success",
  "messageId": "msg_invoice_abc123",
  "timestamp": "2026-07-02T10:30:00Z"
}

Step 2: Send Invoice as PDF Attachment

For PDF invoices, host the PDF and send via /send-document:

curl -X POST https://app.rapiwa.com/send-document \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "number": "8801234567890",
    "documentUrl": "https://yoursite.com/invoices/INV-2026-001.pdf",
    "filename": "Invoice-INV-2026-001.pdf",
    "caption": "Invoice #INV-2026-001 — $1,500 due by July 10, 2026. Pay at: https://pay.yourcompany.com/INV-2026-001"
  }'

Step 3: Invoice + Payment Reminder System in Python

# invoice_notifier.py
# pip install requests schedule psycopg2-binary

import requests
import schedule
import time
import psycopg2
from datetime import datetime, date

RAPIWA_API_KEY = 'YOUR_API_KEY'


def send_whatsapp(phone: str, message: str) -> dict:
    """Send a WhatsApp message via Rapiwa."""
    response = requests.post(
        'https://app.rapiwa.com/send-message',
        headers={'Authorization': f'Bearer {RAPIWA_API_KEY}'},
        json={'number': phone, 'message': message},
        timeout=10
    )
    return response.json()


def send_invoice_notification(phone: str, name: str, invoice_num: str, 
                              amount: float, due_date: str, pay_url: str) -> dict:
    """Send initial invoice notification."""
    message = (
        f"Invoice #{invoice_num}\n\n"
        f"Hi {name}!\n\n"
        f"Your invoice is ready:\n"
        f"*Amount Due:* ${amount:,.2f}\n"
        f"*Due Date:* {due_date}\n\n"
        f"💳 Pay now: {pay_url}\n\n"
        f"Reply with any questions — we're happy to help!"
    )
    return send_whatsapp(phone, message)


def send_payment_reminder(phone: str, name: str, invoice_num: str,
                          amount: float, due_date: str, days_overdue: int,
                          pay_url: str) -> dict:
    """Send a payment reminder (overdue)."""
    if days_overdue <= 0:
        # Pre-due reminder (3 days before)
        urgency = "friendly"
        prefix = f"Quick reminder 😊"
        suffix = "No action needed if you've already paid!"
    elif days_overdue <= 7:
        urgency = "moderate"
        prefix = f"Invoice #{invoice_num} is now *{days_overdue} days overdue*"
        suffix = "Please pay as soon as possible to avoid any late fees."
    else:
        urgency = "urgent"
        prefix = f"⚠️ URGENT: Invoice #{invoice_num} is *{days_overdue} days overdue*"
        suffix = "Please pay immediately or contact us to arrange a payment plan."
    
    message = (
        f"{prefix}\n\n"
        f"Hi {name},\n\n"
        f"*Invoice:* #{invoice_num}\n"
        f"*Amount Due:* ${amount:,.2f}\n"
        f"*Original Due Date:* {due_date}\n\n"
        f"💳 Pay now: {pay_url}\n\n"
        f"{suffix}"
    )
    
    return send_whatsapp(phone, message)


def process_invoice_reminders():
    """
    Query database for invoices needing reminders and send them.
    Run this daily.
    """
    conn = psycopg2.connect("postgresql://user:pass@localhost/billing")
    cursor = conn.cursor()
    
    today = date.today()
    
    # Query unpaid invoices that need reminders
    cursor.execute("""
        SELECT 
            i.id, i.invoice_number, i.amount, i.due_date,
            i.pay_url, i.last_reminder_sent,
            c.phone, c.name
        FROM invoices i
        JOIN customers c ON i.customer_id = c.id
        WHERE i.status = 'unpaid'
          AND c.phone IS NOT NULL
          AND (
            -- 3 days before due (pre-reminder)
            i.due_date = %s + INTERVAL '3 days'
            -- Due today
            OR i.due_date = %s
            -- 3 days overdue (first reminder)
            OR (i.due_date = %s - INTERVAL '3 days' 
                AND (i.last_reminder_sent IS NULL 
                     OR i.last_reminder_sent < %s - INTERVAL '3 days'))
            -- 7 days overdue
            OR (i.due_date = %s - INTERVAL '7 days'
                AND (i.last_reminder_sent IS NULL
                     OR i.last_reminder_sent < %s - INTERVAL '7 days'))
            -- 14 days overdue (final notice)
            OR (i.due_date = %s - INTERVAL '14 days'
                AND (i.last_reminder_sent IS NULL
                     OR i.last_reminder_sent < %s - INTERVAL '14 days'))
          )
    """, [today] * 8)
    
    invoices = cursor.fetchall()
    print(f"Sending {len(invoices)} invoice reminders")
    
    for row in invoices:
        (inv_id, inv_num, amount, due_date, pay_url, 
         last_reminder, phone, name) = row
        
        days_overdue = (today - due_date).days
        
        result = send_payment_reminder(
            phone=phone,
            name=name,
            invoice_num=inv_num,
            amount=float(amount),
            due_date=due_date.strftime('%B %d, %Y'),
            days_overdue=days_overdue,
            pay_url=pay_url
        )
        
        if result.get('status') == 'success':
            cursor.execute(
                "UPDATE invoices SET last_reminder_sent = %s WHERE id = %s",
                [today, inv_id]
            )
    
    conn.commit()
    cursor.close()
    conn.close()


# Schedule: run every day at 9 AM
schedule.every().day.at("09:00").do(process_invoice_reminders)

if __name__ == "__main__":
    print("Invoice reminder service started...")
    process_invoice_reminders()  # Run once immediately
    while True:
        schedule.run_pending()
        time.sleep(60)

Step 4: Node.js Invoice Sender

// invoiceSender.js
// npm install node-fetch

const fetch = require('node-fetch');

const RAPIWA_API_KEY = 'YOUR_API_KEY';

async function sendInvoiceWhatsApp(invoice) {
  const { customerPhone, customerName, invoiceNumber, amount, dueDate, paymentUrl } = invoice;

  const message = `Invoice #${invoiceNumber}\n\n` +
    `Hi ${customerName}! 🧾\n\n` +
    `Your invoice is ready:\n` +
    `*Amount:* $${amount.toFixed(2)}\n` +
    `*Due:* ${dueDate}\n\n` +
    `💳 Pay now: ${paymentUrl}\n\n` +
    `Questions? Just reply here!`;

  const response = await fetch('https://app.rapiwa.com/send-message', {
    method: 'POST',
    headers: {
      'Authorization': `Bearer ${RAPIWA_API_KEY}`,
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({ number: customerPhone, message })
  });

  return response.json();
}

// Usage
sendInvoiceWhatsApp({
  customerPhone: '8801234567890',
  customerName: 'Sarah Johnson',
  invoiceNumber: 'INV-2026-001',
  amount: 1500,
  dueDate: 'July 10, 2026',
  paymentUrl: 'https://pay.yourcompany.com/INV-2026-001'
}).then(result => console.log(result));

Payment Reminder Sequence Best Practices

Common Errors and Fixes

• Customers not responding: WhatsApp messages require an active number. Verify phone numbers are in international format and active
• 401 from Rapiwa: API key expired — regenerate in Dashboard → API Keys
• PDF not opening on WhatsApp mobile: Ensure the PDF URL is publicly accessible and the Content-Type header is application/pdf
• Duplicate reminders sent: Add a last_reminder_sent check in your database before sending

FAQ

Is WhatsApp legal for sending invoices and payment reminders? Yes, with consent. Customers who provide their WhatsApp number for order/billing purposes have implicitly consented to business communications. For colder outreach, add an explicit opt-in at checkout.

Does Rapiwa charge per invoice message? No. Rapiwa charges $5/month flat with no per-message fees. Send 100 or 10,000 invoices at the same monthly cost.

Can I attach a payment button to the WhatsApp message? WhatsApp's official API supports interactive buttons (payment, quick reply) but only through Meta's official WhatsApp Business Platform. With Rapiwa (unofficial API), you can include a payment link in text — customers click it to pay.

How do I handle customers who pay by cheque and not online? Add a "Reply PAID if you've paid by cheque/bank transfer" option. When they reply PAID, your webhook processor updates the invoice status.

Can I integrate this with Xero, QuickBooks, or FreshBooks? Yes. These tools have webhook APIs — when an invoice is created in Xero/QuickBooks, trigger the WhatsApp notification via n8n or a custom webhook handler.