PayPal IPN returns INVALID on Sandbox with Google Script - paypal-ipn

Question

This standalone Google Script web-service always returns INVALID from PayPal (resp = INVALID). The PayPal IPN simulator shows a message that the handshake was successful.
What am I missing?
function doPost(e) {
var isProduction = false;
//if(typeof e == 'undefined')
//return ContentService.createTextOutput(JSON.stringify(e.parameter));
var strSandbox = "https://www.sandbox.paypal.com/cgi-bin/webscr"
var strLive = "https://www.paypal.com/cgi-bin/webscr"
var paypalURL = strSandbox
if (isProduction) paypalURL = strLive;
var payload = "cmd=_notify-validate&" + e.postData.contents;
var options =
{
"method" : "post",
"payload" : payload
};
var resp = UrlFetchApp.fetch(paypalURL, options);
}

Answer 1

After getting everything to work, I'm offering here my working implementation skeleton:
function doPost(e) {
var isProduction = false;
//if(typeof e == 'undefined')
//return ContentService.createTextOutput(JSON.stringify(e.parameter));
var strSimulator = "https://www.sandbox.paypal.com/cgi-bin/webscr"
var strLive = "https://www.paypal.com/cgi-bin/webscr"
var paypalURL = strSimulator
if (isProduction) paypalURL = strLive;
var payload = "cmd=_notify-validate&" + e.postData.contents;
payload = payload.replace("+", "%2B");
var options =
{
"method" : "post",
"payload" : payload,
};
var resp = UrlFetchApp.fetch(paypalURL, options); //Handshake with PayPal - send acknowledgement and get VERIFIED or INVALID response
if (resp == 'VERIFIED') {
if (e.parameter.payment_status == 'Completed') {
if (e.parameter.receiver_email == 'receiver#email.com') {
//Convert to reference currency (USD) if paid in any other currency
var exchangeRate = 1;
if ((e.parameter.exchange_rate)) {
exchangeRate = parseFloat(e.parameter.exchange_rate);
}
var paidUSD = isPaymentValid(parseFloat(e.parameter.mc_gross), e.parameter.mc_currency, exchangeRate); //Convert paid amound to reference currency (USD)
if (paidUSD == 0.0) {
//My function returns 0.0 if product cost not found in my DB. I raise some notification here to check it out
return false;
}
if (paidUSD > 0.0) {
//All validated - can process the payment
var processSuccess = processDownloadRequest(e);
if (!(processSuccess)) {
//Process of payment failed - raise notification to check it out
}
} else {
//Payment does not equal expected purchase value
}
} else {
//Request did not originate from my PayPal account
}
} else {
//Payment status not Completed
}
} else
{
//PayPal response INVALID
}
}