Confirm Checkout Session

curl --request POST \
  --url https://api.withflex.com/v1/checkout/sessions/{id}/confirm \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --header 'Idempotency-Key: <idempotency-key>' \
  --data '
{
  "checkout_session": {
    "payment_method": "fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "capture_method": "automatic"
  }
}
'

import requests

url = "https://api.withflex.com/v1/checkout/sessions/{id}/confirm"

payload = { "checkout_session": {
        "payment_method": "fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
        "capture_method": "automatic"
    } }
headers = {
    "Idempotency-Key": "<idempotency-key>",
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

response = requests.post(url, json=payload, headers=headers)

print(response.text)

const options = {
  method: 'POST',
  headers: {
    'Idempotency-Key': '<idempotency-key>',
    Authorization: 'Bearer <token>',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    checkout_session: {payment_method: 'fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA', capture_method: 'automatic'}
  })
};

fetch('https://api.withflex.com/v1/checkout/sessions/{id}/confirm', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.withflex.com/v1/checkout/sessions/{id}/confirm",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => json_encode([
    'checkout_session' => [
        'payment_method' => 'fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA',
        'capture_method' => 'automatic'
    ]
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: Bearer <token>",
    "Content-Type: application/json",
    "Idempotency-Key: <idempotency-key>"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://api.withflex.com/v1/checkout/sessions/{id}/confirm"

	payload := strings.NewReader("{\n  \"checkout_session\": {\n    \"payment_method\": \"fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA\",\n    \"capture_method\": \"automatic\"\n  }\n}")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Idempotency-Key", "<idempotency-key>")
	req.Header.Add("Authorization", "Bearer <token>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

HttpResponse<String> response = Unirest.post("https://api.withflex.com/v1/checkout/sessions/{id}/confirm")
  .header("Idempotency-Key", "<idempotency-key>")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "application/json")
  .body("{\n  \"checkout_session\": {\n    \"payment_method\": \"fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA\",\n    \"capture_method\": \"automatic\"\n  }\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://api.withflex.com/v1/checkout/sessions/{id}/confirm")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Idempotency-Key"] = '<idempotency-key>'
request["Authorization"] = 'Bearer <token>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"checkout_session\": {\n    \"payment_method\": \"fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA\",\n    \"capture_method\": \"automatic\"\n  }\n}"

response = http.request(request)
puts response.read_body

{
  "checkout_session": {
    "allow_promotion_codes": false,
    "amount_total": 2500,
    "amount_subtotal": 2500,
    "amount_received": 2500,
    "amount_refunded": 2500,
    "amount_disputed": 2500,
    "cancel_url": "https://example.com",
    "captures": [],
    "disputes": [],
    "capture_method": "automatic",
    "checkout_session_id": "fcs_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "client_reference_id": "example_value",
    "created_at": 2500,
    "defaults": {},
    "expires_at": 2500,
    "invoice": "fin_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "hsa_fsa_eligible": false,
    "letter_of_medical_necessity_required": false,
    "metadata": {
      "order_id": "8842",
      "channel": "shopify"
    },
    "mode": "payment",
    "origin": "shopify",
    "payment_intents": [],
    "redirect_url": "https://example.com",
    "refunds": [],
    "setup_intent": "fseti_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "shipping_options": {},
    "shipping_address_collection": false,
    "shipping_details": {},
    "status": "open",
    "success_url": "https://example.com",
    "fees": [
      {}
    ],
    "custom_text": {},
    "subscription_data": {},
    "tax_rate": {},
    "tax_calculation_mode": "exclusive",
    "test_mode": false,
    "total_details": {
      "amount_discount": 2500,
      "amount_tax": 2500,
      "amount_shipping": 2500,
      "amount_iias": 2500,
      "amount_vision": 2500,
      "amount_prescription": 2500,
      "amount_service": 2500,
      "amount_fee": 2500
    },
    "visit_type": "cbtSleep",
    "setup_future_use": "on_session",
    "payment_method_options": {},
    "platform_fees": {}
  }
}

{
  "code": "string",
  "detail": "string"
}

{
  "code": "string",
  "detail": "string"
}

{
  "code": "string",
  "detail": "string"
}

{
  "code": "string",
  "detail": "string"
}

{
  "detail": [
    {
      "loc": [
        "string"
      ],
      "msg": "string",
      "type": "string"
    }
  ]
}

{
  "code": "string",
  "detail": "string"
}

Confirm Checkout Session

Charges the HSA/FSA-eligible portion of a Checkout Session against a single saved payment method, using the eligibility split computed and persisted when the session was created. The amount charged is the sum of the eligible portions of the session’s items — never the session total — so amount_received is expected to be less than amount_total. Collect the remaining ineligible balance through your own payment flow.

Returns the Checkout Session with its payment_intent expanded. When the eligible portion sums to $0, or to less than Stripe’s $0.50 minimum, nothing is charged and the response is still a 200 with payment_intent: null — a normal outcome for a partly-eligible order, not an error. Branch on payment_intent == null to decide whether there is anything to settle with Flex.

capture_method must be automatic. manual is rejected with manual_capture_not_supported (422): it would authorize the card successfully and then be uncapturable, so it is refused before any money moves rather than leaving a hold to expire.

An Idempotency-Key header is required. Retry a timeout or a 5xx with the same key; a new key is a new request and can charge the card a second time. A 4xx and a card decline are both terminal.

Error codes: idempotency_key_required (400), checkout_session_already_processed / checkout_session_expired / checkout_session_canceled (400), checkout_session_not_found / payment_method_not_found / customer_not_found / eligibility_record_not_found / food_eligibility_not_enabled (404), eligibility_record_already_charged / idempotency_key_reused (409), manual_capture_not_supported / payment_method_not_off_session (422), idempotency_key_in_use / lock_timeout (503 — transient contention: a request with this key is still in flight, or another request holds the customer or payment method. Retry unchanged with the same key; a new key may charge the card twice), and a 402 carrying code and decline_code for a card decline — including authentication_required, which is terminal: a card demanding 3DS cannot be charged off-session, so fall back to another tender rather than retrying.

POST

checkout

sessions

{id}

confirm

Confirm Checkout Session

curl --request POST \
  --url https://api.withflex.com/v1/checkout/sessions/{id}/confirm \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --header 'Idempotency-Key: <idempotency-key>' \
  --data '
{
  "checkout_session": {
    "payment_method": "fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "capture_method": "automatic"
  }
}
'

import requests

url = "https://api.withflex.com/v1/checkout/sessions/{id}/confirm"

payload = { "checkout_session": {
        "payment_method": "fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
        "capture_method": "automatic"
    } }
headers = {
    "Idempotency-Key": "<idempotency-key>",
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

response = requests.post(url, json=payload, headers=headers)

print(response.text)

const options = {
  method: 'POST',
  headers: {
    'Idempotency-Key': '<idempotency-key>',
    Authorization: 'Bearer <token>',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    checkout_session: {payment_method: 'fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA', capture_method: 'automatic'}
  })
};

fetch('https://api.withflex.com/v1/checkout/sessions/{id}/confirm', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.withflex.com/v1/checkout/sessions/{id}/confirm",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => json_encode([
    'checkout_session' => [
        'payment_method' => 'fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA',
        'capture_method' => 'automatic'
    ]
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: Bearer <token>",
    "Content-Type: application/json",
    "Idempotency-Key: <idempotency-key>"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://api.withflex.com/v1/checkout/sessions/{id}/confirm"

	payload := strings.NewReader("{\n  \"checkout_session\": {\n    \"payment_method\": \"fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA\",\n    \"capture_method\": \"automatic\"\n  }\n}")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Idempotency-Key", "<idempotency-key>")
	req.Header.Add("Authorization", "Bearer <token>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

HttpResponse<String> response = Unirest.post("https://api.withflex.com/v1/checkout/sessions/{id}/confirm")
  .header("Idempotency-Key", "<idempotency-key>")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "application/json")
  .body("{\n  \"checkout_session\": {\n    \"payment_method\": \"fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA\",\n    \"capture_method\": \"automatic\"\n  }\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://api.withflex.com/v1/checkout/sessions/{id}/confirm")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Idempotency-Key"] = '<idempotency-key>'
request["Authorization"] = 'Bearer <token>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"checkout_session\": {\n    \"payment_method\": \"fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA\",\n    \"capture_method\": \"automatic\"\n  }\n}"

response = http.request(request)
puts response.read_body

{
  "checkout_session": {
    "allow_promotion_codes": false,
    "amount_total": 2500,
    "amount_subtotal": 2500,
    "amount_received": 2500,
    "amount_refunded": 2500,
    "amount_disputed": 2500,
    "cancel_url": "https://example.com",
    "captures": [],
    "disputes": [],
    "capture_method": "automatic",
    "checkout_session_id": "fcs_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "client_reference_id": "example_value",
    "created_at": 2500,
    "defaults": {},
    "expires_at": 2500,
    "invoice": "fin_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "hsa_fsa_eligible": false,
    "letter_of_medical_necessity_required": false,
    "metadata": {
      "order_id": "8842",
      "channel": "shopify"
    },
    "mode": "payment",
    "origin": "shopify",
    "payment_intents": [],
    "redirect_url": "https://example.com",
    "refunds": [],
    "setup_intent": "fseti_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "shipping_options": {},
    "shipping_address_collection": false,
    "shipping_details": {},
    "status": "open",
    "success_url": "https://example.com",
    "fees": [
      {}
    ],
    "custom_text": {},
    "subscription_data": {},
    "tax_rate": {},
    "tax_calculation_mode": "exclusive",
    "test_mode": false,
    "total_details": {
      "amount_discount": 2500,
      "amount_tax": 2500,
      "amount_shipping": 2500,
      "amount_iias": 2500,
      "amount_vision": 2500,
      "amount_prescription": 2500,
      "amount_service": 2500,
      "amount_fee": 2500
    },
    "visit_type": "cbtSleep",
    "setup_future_use": "on_session",
    "payment_method_options": {},
    "platform_fees": {}
  }
}

{
  "code": "string",
  "detail": "string"
}

{
  "code": "string",
  "detail": "string"
}

{
  "code": "string",
  "detail": "string"
}

{
  "code": "string",
  "detail": "string"
}

{
  "detail": [
    {
      "loc": [
        "string"
      ],
      "msg": "string",
      "type": "string"
    }
  ]
}

{
  "code": "string",
  "detail": "string"
}

Authorizations

Authorization

string

header

required

Use a Bearer token to access this API.

Headers

Idempotency-Key

string

required

A unique key identifying this attempt. Retry a timed-out or 5xx request with the SAME key — a new key is a new request and can charge the card a second time.

Path Parameters

string

required

Example:

"fcs_01J9XR8M3K7VZ8N2YB4WJ6T0RA"

Query Parameters

expand

string

default:""

Comma-separated list of related objects to expand in the response.

Example:

""

Body

application/json

An envelope wrapping a single checkout session object.

checkout_session

object

required

The checkout session payload.

Show child attributes

Example:

{
  "payment_method": "fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
  "capture_method": "automatic"
}

Response

An envelope wrapping a single checkout session object.

checkout_session

object

required

The checkout session payload.

Show child attributes

Example:

{
  "allow_promotion_codes": false,
  "amount_total": 2500,
  "amount_subtotal": 2500,
  "amount_received": 2500,
  "amount_refunded": 2500,
  "amount_disputed": 2500,
  "cancel_url": "https://example.com",
  "captures": ["fcap_01J9XR8M3K7VZ8N2YB4WJ6T0RA"],
  "disputes": ["fdp_01J9XR8M3K7VZ8N2YB4WJ6T0RA"],
  "capture_method": "automatic",
  "checkout_session_id": "fcs_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
  "client_reference_id": "example_value",
  "created_at": 2500,
  "customer": "fcus_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
  "defaults": {
    "customer_id": "fcus_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "email": "john.doe@example.com",
    "first_name": "John",
    "last_name": "Doe",
    "phone": "+14155550142"
  },
  "expires_at": 2500,
  "invoice": "fin_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
  "hsa_fsa_eligible": false,
  "letter_of_medical_necessity_required": false,
  "metadata": { "order_id": "8842", "channel": "shopify" },
  "mode": "payment",
  "origin": "shopify",
  "payment_intent": "fpi_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
  "payment_intents": ["fpi_01J9XR8M3K7VZ8N2YB4WJ6T0RA"],
  "redirect_url": "https://example.com",
  "refunds": ["fre_01J9XR8M3K7VZ8N2YB4WJ6T0RA"],
  "setup_intent": "fseti_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
  "shipping_options": {
    "shipping_rate_id": "fshr_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "shipping_amount": 2500,
    "display_name": "Example"
  },
  "shipping_address_collection": false,
  "shipping_details": {
    "shipping_address_id": "fsh_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
    "line1": "123 Main St",
    "line2": "Suite 100",
    "city": "San Francisco",
    "state": "CA",
    "postal_code": "94107",
    "country": "US"
  },
  "split_cart": "fsplit_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
  "status": "open",
  "success_url": "https://example.com",
  "fees": [
    {
      "fee_id": "ffee_1234567890",
      "amount": 2500,
      "name": "Example",
      "description": "Premium SPF 50 mineral sunscreen",
      "fee_type": "lmn_consultation",
      "applied": true
    }
  ],
  "custom_text": {
    "terms_of_service_checkbox_required": true
  },
  "subscription": "fsub_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
  "subscription_data": {
    "cancel_at": "2026-06-15T14:30:00Z",
    "cancel_at_period_end": false,
    "trial_period_days": 0
  },
  "tax_rate": { "amount": 2500 },
  "tax_calculation_mode": "exclusive",
  "test_mode": false,
  "total_details": {
    "amount_discount": 2500,
    "amount_tax": 2500,
    "amount_shipping": 2500,
    "amount_iias": 2500,
    "amount_vision": 2500,
    "amount_prescription": 2500,
    "amount_service": 2500,
    "amount_fee": 2500,
    "breakdown": {}
  },
  "visit_type": "cbtSleep",
  "setup_future_use": "on_session",
  "payment_method_options": {},
  "next_action": {
    "type": "collect_letter_of_medical_necessity"
  },
  "partner": "facct_01J9XR8M3K7VZ8N2YB4WJ6T0RA",
  "platform_fees": { "letter_fee": {} }
}

⌘I

API Overview

API Reference

Confirm Checkout Session

Authorizations

Headers

Path Parameters

Query Parameters

Body

Response