At this point you should have a Flex account, access to the partner dashboard, and have gone through the onboarding process of adding your payout information with Stripe.

Below we walk through a full example of how to integrate Flex Checkout into your checkout flow.

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Flex Checkout guide

1

Create an API Key in the partner dashboard

If you’re just getting started, be sure to create an API Key in test mode to safely distinguish between testing and production.

In the partner dashboard, navigate to the Developers section. Here you’ll find an API Keys tab where you can create a key to interact with Flex.

Select the New API Key button and give your key a name. With this key, you can now make authenticated requests to the Flex API.

2

Define a product to sell

Always keep sensitive information—like price and availability—on your server to prevent customer-side manipulation. Define product data when creating the Checkout Session using predefined price IDs or on the fly with price_data.

For this example, we’ve created a product eligible via a Letter of Medical Necessity. To learn more, see the Product docs or the Product API.

curl --request POST \
  --url https://api.withflex.com/v1/products \
  --header 'authorization: Bearer fsk_test_YzNiYjdhNWItN2FkOS00ZTMyLWE5MmQtZTdhNzMzYzE2NTIy' \
  --header 'content-type: application/json' \
  --data '{"product": {"name": "Test Product","description": "Test Product Description","visit_type": "gym","hsa_fsa_eligibility": "letter_of_medical_necessity"}}'

{
  "product": {
    "product_id": "fprod_01HW5MXAPBE79RHMMJJGB4ACAB",
    "name": "Test Product",
    "description": "Test Product Description",
    "created_at": "2024-04-23T14:14:16.029253Z",
    "visit_type": "gym",
    "active": true,
    "upc_code": null,
    "gtin": null,
    "hsa_fsa_eligibility": "letter_of_medical_necessity"
  }
}
3

Define the price for our product

Using the returned product_id, create a one-time price for $50.99:

curl --request POST \
  --url https://api.withflex.com/v1/prices \
  --header 'authorization: Bearer fsk_test_YzNiYjdhNWItN2FkOS00ZTMyLWE5MmQtZTdhNzMzYzE2NTIy' \
  --header 'content-type: application/json' \
  --data '{"price": {"product": "fprod_01HW5MXAPBE79RHMMJJGB4ACAB","description": "Our awesome new price","unit_amount": 5099}}'

{
  "price": {
    "price_id": "fprice_01HW5NTAB88NK7HPD0H688EPG9",
    "description": "Our awesome new price",
    "unit_amount": 5099,
    "recurring": null,
    "active": true,
    "product": "fprod_01HW5MXAPBE79RHMMJJGB4ACAB",
    "created_at": "2024-04-23T14:30:05.943282Z",
    "type": "one_time",
    "metadata": null
  }
}
4

Choose the mode

Checkout supports two modes: payment (one-time purchases) or subscription. Learn more about subscription and setup modes in the docs.

5

Supply success and cancel URLs

Specify publicly accessible URLs so Flex can redirect after checkout:

  • success_url: https://withflex.com/thank-you?success=true
  • cancel_url: https://withflex.com/thank-you?canceled=true
6

Build your checkout

On your site’s order preview page, add a button that calls your server to generate a checkout session.

7

Generate a checkout session

On your server, create a session with your line items and redirect URLs:

curl --request POST \
  --url https://api.withflex.com/v1/checkout/sessions \
  --header 'authorization: Bearer fsk_test_YzNiYjdhNWItN2FkOS00ZTMyLWE5MmQtZTdhNzMzYzE2NTIy' \
  --header 'content-type: application/json' \
  --data '{
    "checkout_session": {
      "line_items": [{"price": "fprice_01HW5NTAB88NK7HPD0H688EPG9","quantity": 1}],
      "success_url": "https://withflex.com/thank-you?success=true",
      "cancel_url": "https://withflex.com/thank-you?canceled=true",
      "mode": "payment"
    }
  }'
8

Redirect users to checkout

The response includes a redirect_url—send the user there to complete payment:

{
  "checkout_session": {
    "redirect_url": "https://checkout.withflex.com/pay/c/fcs_01HW5Q2A08N6A0YGZ6T1KARN8S",
    …
  }
}
9

Try it out

Navigate to the returned redirect_url and test with these cards:

  • Success: 4242 4242 4242 4242
  • Decline: 4000 0000 0000 9995
10

Congratulations!

You now have a basic checkout integration working 🎉

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Fulfillment

Set up a webhook to fulfill orders after a payment succeeds.

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Explore checkout session scenarios

Depending on your setup/business you may want to rely on additional parameters that are available as part of the Checkout Session API. Some of the common scenarios, can be found here.