Fulfill Orders - Flex Documentation

After you integrate with Flex Checkout or create a Payment Link to take your customers to a payment form, you need a notification to fulfill their order after a successful payment. In this guide, you’ll learn how to use webhooks to fulfill orders after a payment.

Fulfillment guide

Select webhook events

Choose the webhook events that are relevant to your checkout session.

customer.created — When a customer record is created in Flex.
payment_intent.created — As soon as the customer begins entering payment info.
payment_intent.succeeded — After a successful payment (fires twice on split carts).
customer.subscription.created — For subscription sessions, when a subscription is first created. Includes the subscription’s initial status (e.g., trialing, active, or incomplete).
customer.subscription.updated — Sent when an existing subscription changes (plan changes, payment failure, cancellation, etc.). Not sent at subscription creation, and not reliably sent on trialing → active transitions. Don’t depend on this event for detecting new subscriptions.
invoice.payment_succeeded — Sent when a subscription invoice is paid. The most reliable signal that a customer has paid — recommended for provisioning or extending membership / entitlements.
checkout_session.completed — Sent after a checkout session has fully completed.

Recommended pattern for subscription fulfillmentTo avoid missing new subscriptions, follow this pattern:

Provision membership / access on customer.subscription.created, gated on status in (trialing, active).
Confirm or extend entitlement on invoice.payment_succeeded (covers initial payment and renewals).
Sync state (status changes, plan changes, payment failures) on customer.subscription.updated.
Revoke access on customer.subscription.deleted.

Listening only on customer.subscription.updated will cause you to silently miss new subscriptions, since Stripe does not fire .updated at sub creation.

Create an event handler

Set up an HTTP endpoint to receive and log incoming events. For local testing, use ngrok or webhook.site.

// Using Express
const app = require("express")();
const bodyParser = require("body-parser");

app.post(
  "/webhook",
  bodyParser.raw({ type: "application/json" }),
  (req, res) => {
    console.log("Got payload:", req.body);
    res.status(200).end();
  }
);

app.listen(4242, () => console.log("Running on port 4242"));

Create a webhook endpoint

curl --request POST \
  --url https://api.withflex.com/v1/webhooks \
  --header 'authorization: Bearer fsk_test_…' \
  --header 'content-type: application/json' \
  --data '{
    "webhook": {
      "url": "https://withflex.com/handle_events",
      "events": ["checkout.session.completed"]
    }
  }'

Create a checkout session

Trigger a session to confirm your handler receives events:

curl --request POST \
  --url https://api.withflex.com/v1/checkout/sessions \
  --header 'authorization: Bearer fsk_test_…' \
  --header 'content-type: application/json' \
  --data '{
    "checkout_session": {
      "line_items": [{"price": "fprice_…","quantity": 1}],
      "success_url": "https://withflex.com/thank-you?success=true",
      "cancel_url": "https://withflex.com/thank-you?canceled=true",
      "mode": "payment"
    }
  }'

Then go through checkout:

Enter contact info
Fill any intake form
Use card number 4242 4242 4242 4242, future expiry, any CVV, and postal code 90210
Click Pay

You should see an event payload like:

{
  "event_type": "checkout.session.completed",
  "object": {
    "checkout_session": {
      "status": "complete",
      "redirect_url": "https://checkout.withflex.com/pay/…",
      …
    }
  }
}

Verify events came from Flex

Confirm the signature on incoming requests before processing. See Verifying webhooks.

Fulfill order

Handle checkout.session.completed to:

Save the order to your database
Send a receipt email
Update inventory via the Line Items API:

curl --request GET \
  --url https://api.withflex.com/v1/checkout/sessions/fcs_…/line_items \
  --header 'authorization: Bearer fsk_test_…'

Go live in production

Once deployed, register your production webhook URL in Flex.

​Fulfillment guide