Flex Checkout Quick Start

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.

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"
  }
}

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
  }
}

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

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

On your site’s order preview page, add a button that calls your server to 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"
    }
  }'

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",
    …
  }
}

Navigate to the returned redirect_url and test with these cards:

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

You now have a basic checkout integration working 🎉