diff --git a/javascript/firebase.js b/javascript/firebase.js
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+/**
+ * Sample PayPal IPN Listener implemented for Firebase Functions.
+ */
+
+const axios = require('axios');
+const functions = require('firebase-functions');
+
+/**
+ * @const {boolean} sandbox Indicates if the sandbox endpoint is used.
+ */
+const sandbox = true;
+
+/** Production Postback URL */
+const PRODUCTION_VERIFY_URI = 'https://ipnpb.paypal.com/cgi-bin/webscr';
+/** Sandbox Postback URL */
+const SANDBOX_VERIFY_URI = 'https://ipnpb.sandbox.paypal.com/cgi-bin/webscr';
+
+/**
+ * Determine endpoint to post verification data to.
+ *
+ * @return {String}
+ */
+function getPaypalURI() {
+  return sandbox ? SANDBOX_VERIFY_URI : PRODUCTION_VERIFY_URI;
+}
+
+/**
+ * @param {Object} req Firebase Function request context for IPN notification event.
+ * @param {Object} res Firebase Function response context.
+ */
+exports.ipnHandler = functions.https.onRequest(async (req, res) => {
+  console.log('IPN Notification Event Received');
+
+  if (req.method !== 'POST') {
+    console.error('Request method not allowed.');
+    res.status(405).send('Method Not Allowed');
+  } else {
+    // Return empty 200 response to acknowledge IPN post success.
+    console.log('IPN Notification Event received successfully.');
+    res.status(200).end();
+  }
+
+  // JSON object of the IPN message consisting of transaction details.
+  const ipnTransactionMessage = req.body;
+
+  // Build the body of the verification post message by prefixing 'cmd=_notify-validate'.
+  const verificationBody = `cmd=_notify-validate&${req.rawBody}`;
+
+  console.log(`Verifying IPN: ${verificationBody}`);
+
+  const verifyResponse = await axios({
+    method: 'post',
+    url: getPaypalURI(),
+    data: verificationBody,
+  });
+
+  if (verifyResponse.status !== 200) {
+    console.log(
+      `Invalid IPN: IPN message for ID: ${ipnTransactionMessage.txn_id} failed with code ${verifyResponse.status}`
+    );
+
+    return;
+  }
+
+  if (verifyResponse.data !== 'VERIFIED') {
+    console.error(
+      `Invalid IPN: Message for ID: ${ipnTransactionMessage.txn_id} is invalid (${verifyResponse.data}).`,
+    );
+
+    return;
+  }
+
+  console.log(
+    `Verified IPN: IPN message for ID: ${ipnTransactionMessage.txn_id} was verified.`
+  );
+
+  // TODO: Implement your custom post verification logic to handle this event here
+});