integration_script.js

// Order confirmation page aka post purchase
if (Shopify && Shopify.checkout) {
  var _checkout = Shopify.checkout;

  _q.push(['register_purchase', {
    email: _checkout.email,
    order_number: _checkout.order_id,
    subtotal: _checkout.total_price,
    coupon_code: _checkout.discount && _checkout.discount.applicable ? _checkout.discount.code : null
  }]);
} else {
  // some non post-purchase stuff goes here that is loaded on every page.
  // I removed it to focus attention on the code snippet under question.
}