Overdue payment reminder via WhatsApp API

The scenario sends the customer a personalised WhatsApp template when their payment becomes overdue.

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

{{1}}
customer name
{{2}}
outstanding payment amount
{{3}}
overdue payment date
“Pay”
button — fixed in the Meta template

WhatsApp Business API for overdue payment reminder

When to use it

Reach for this scenario when billing marks an invoice as past due and reminders by email or SMS are getting overlooked while manual collection calls strain the team. It fits subscription billing, B2B services with outstanding contract balances, and agencies connecting ERP or accounting systems to automated WhatsApp payment nudges.

Fewer overlooked overdue notices

Customer name, outstanding amount and overdue date arrive as three body variables in WhatsApp, so the reminder stays visible in a chat they check instead of buried in email.

One-tap path to pay

The static URL button fixed in the approved Meta template opens the online payment page, so the customer can complete payment without replying in chat.

Less manual collection work

Billing or accounting detects the past-due event and sends the template automatically, so no one retypes amounts and dates for each overdue account.

Delivery logged for follow-up

Delivery result is logged for billing or CRM, so collection workflows know whether the reminder reached the customer before the next escalation step.

Workflow

Trigger
Billing or the accounting system detects that a payment is past due.
event · triggered
Capture event
Customer phone number, outstanding amount, and overdue date are retrieved from the billing system.
phone: "+…"
Build & send
A personalised template message is built with three body variables and a static URL button linking to the payment page.
POST /sendTemplate
Delivered
The customer receives the WhatsApp reminder and can follow the link to complete payment.
delivered
Status tracked
Delivery result is logged for further collection or CRM follow-up.
status: "read"

Technical implementation

Prerequisites

1MSG API Key · How to get API Key
WhatsApp Business account · How to Connect WABA
WhatsApp Template · How to Approve WABA Template
Customer opt-in · How to Manage Customers Consent

Code examples

#!/usr/bin/env bash
set -euo pipefail

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

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

TEMPLATE_NAME="___"                        # approved template name
TEMPLATE_NAMESPACE="___"                   # template namespace (required — send fails without it)
TEMPLATE_LANGUAGE="___"                    # template language code, e.g. "en"



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

PHONE_NORM="$(printf '%s' "$TEST_PHONE" | tr -cd '0-9')"

for pair in "CHANNEL_ID=$CHANNEL_ID" "API_TOKEN=$API_TOKEN" \
            "TEMPLATE_NAME=$TEMPLATE_NAME" "TEMPLATE_NAMESPACE=$TEMPLATE_NAMESPACE" \
            "TEMPLATE_LANGUAGE=$TEMPLATE_LANGUAGE" "TEST_PHONE=$TEST_PHONE" \
            "TEST_CUSTOMERNAME=$TEST_CUSTOMERNAME" \
            "TEST_PAYMENTAMOUNT=$TEST_PAYMENTAMOUNT" \
            "TEST_DUEDATE=$TEST_DUEDATE"; do
    val="${pair#*=}"
    if [ -z "$val" ] || [ "$val" = "___" ]; then
        echo "Missing configuration value: ${pair%%=*}" >&2
        exit 1
    fi
done

if [ -z "$PHONE_NORM" ]; then
    echo "Error: phone number has no digits after normalization" >&2
    exit 1
fi

URL="${API_BASE_URL%/}/${CHANNEL_ID}/sendTemplate"

# params carries body ONLY. Button text is fixed in the Meta template — no button param.

# {{1}} customer name → ${TEST_CUSTOMERNAME}
# {{2}} payment amount → ${TEST_PAYMENTAMOUNT}
# {{3}} due date → ${TEST_DUEDATE}
read -r -d '' PAYLOAD <<JSON || true
{
  "phone": "${PHONE_NORM}",
  "template": "${TEMPLATE_NAME}",
  "namespace": "${TEMPLATE_NAMESPACE}",
  "language": { "policy": "deterministic", "code": "${TEMPLATE_LANGUAGE}" },
  "params": [
    {
      "type": "body",
      "parameters": [
        { "type": "text", "text": "${TEST_CUSTOMERNAME}" },
        { "type": "text", "text": "${TEST_PAYMENTAMOUNT}" },
        { "type": "text", "text": "${TEST_DUEDATE}" }
      ]
    }

  ]
}
JSON

RESPONSE="$(curl -s -w '\n%{http_code}' -X POST "$URL" \
    -H "Content-Type: application/json" \
    -H "Authorization: Bearer ${API_TOKEN}" \
    -d "$PAYLOAD")"

HTTP_CODE="$(printf '%s' "$RESPONSE" | tail -n1)"
BODY="$(printf '%s' "$RESPONSE" | sed '$d')"

case "$BODY" in
    *'"sent":true'*) ok=1 ;;
    *) ok=0 ;;
esac

if [ "$HTTP_CODE" -ge 200 ] && [ "$HTTP_CODE" -lt 300 ] && [ "$ok" -eq 1 ]; then
    echo "Message sent to client."
    echo "API response: $BODY"
else
    echo "Send failed. HTTP status: $HTTP_CODE" >&2
    echo "$BODY" >&2
    exit 1
fi

Response and delivery status

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 (looks like wamid.…) to correlate delivery callbacks.

200 OKResponse

{
  "sent": true,
  "message": "Sent to [email protected]",
  "description": "Message has been sent to the provider",
  "id": "wamid.HBgLMzgwNjM5..."
}

sent
Accepted for sending — not yet on the customer's phone
id
Store it; delivery callbacks and hookInfo are keyed on this

Delivery statuses and webhooks →

Common errors

Status	API response	Cause	Fix
200	Message was not sent: template is not defined	namespace, template or language is missing from the request body.	Send all three. Take namespace and the exact template name from GET /templates; language is an object: {"policy": "deterministic", "code": "en"}.
200	template name (…) does not exist in <language>	The template is approved in a different language than the one requested.	Use the exact language code the template was approved in (for example es_MX is not the same as es). Check it in GET /templates.
200	Message was not sent: provide chatId, phone, bsuid, or username	No recipient the channel could resolve.	Pass exactly one recipient: phone (country code plus number, digits only), chatId (for example [email protected]) or bsuid.