I'm following the documentation for Stripe Checkout with server integration: <https://stripe.com/docs/payments/checkout/server>

The code in the examples works fine, but the problem I'm having is being unable to track the user, or their order, when a purchase is complete.

I have a webhook setup that Stripe pings when a payment is complete. But the response session from Stripe contains no information about the product ordered except for it's name, description, and image. I could use the product name to query the database but I'd much rather an ID, or a slug, of sorts.

```
$app->post("/stripe-pingback", function(Request $request, Response $response, array $args) {
    \Stripe\Stripe::setApiKey("xxxxx");

    // You can find your endpoint's secret in your webhook settings
    $endpoint_secret = 'xxxxx';

    $payload = $request->getBody();
    $sig_header = isset($_SERVER['HTTP_STRIPE_SIGNATURE']) ? $_SERVER['HTTP_STRIPE_SIGNATURE'] : null;
    $event = null;

    try {
        $event = \Stripe\Webhook::constructEvent($payload, $sig_header, $endpoint_secret);
    } catch(\UnexpectedValueException $e) {
        // Invalid payload
        http_response_code(400); // PHP 5.4 or greater
        exit();
    } catch(\Stripe\Error\SignatureVerification $e) {
        // Invalid signature
        http_response_code(400); // PHP 5.4 or greater
        exit();
    }

    // Handle the checkout.session.completed event
    if ($event->type == 'checkout.session.completed') {
        $session = $event->data->object;
        var_dump($session);

        // Possible to get custom data from session?
        $customer = $session->customer;
        $customerEmail = $session->customer_email;

        // Fulfill the purchase...
        $this->db->insertAudioPurchase();
    }

    http_response_code(200); // PHP 5.4 or greater
});

```

Is it possible to pass an ID along with the checkout request that Stripe can ping back to allow me to lookup the order and generate a download link?