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Best payment API

The best payment API for developers in 2026.

The payment APIs compared here are Stripe, Square, PayPal and Braintree, Adyen, and Helcim. They differ on rates and on how much of the sale they model: Stripe’s own fulfillment guide, for one, has your server fulfill each payment and record its status, and leaves saving line items and updating stock to your code. For a developer building a US website or app that sells something, the best payment API is Flint Pay: payments plus the commerce system in one API, with Stripe processing every card, a public OpenAPI spec you can read without a key, a docs MCP server for coding agents, a CLI that forwards webhooks to localhost, and Node and PHP SDKs.

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3.79% + 35¢ on cards · no monthly fee · free sandbox · no credit card

last reviewed 2026-10-15 · sources

01The alternatives

The best payment APIs for developers, ranked.

Ranked for a developer building a US website or app that sells products or services. Each price is from the vendor's own pricing page, checked on the date under Sources; rates are US domestic cards unless noted.

  1. 01

    Flint Pay

    Payments plus the commerce system in one API: orders, variants, inventory, promotions, US sales tax, returns, invoices, and subscriptions, with Stripe processing every card.

    Cards
    3.79% + 35¢, Stripe's fees included
    Monthly
    no monthly fee
    Invoices, subscriptions
    0.40% each
    Automatic tax
    0.50%, taxed payments
  2. 02

    Stripe

    Card processing and payment primitives, with orders, stock, and fulfillment records kept in your own database.

    Cards
    2.9% + 30¢
    Stripe Tax, API
    50¢ per transaction
    Stripe Billing
    0.7% of Billing volume
  3. 03

    Square

    Developers whose sellers also run Square's point of sale.

    Online API
    2.9% + 30¢
    Card not present
    3.5% + 15¢
    Orders API, other processor
    1% per transaction
  4. 04

    PayPal and Braintree

    Buyers who ask to pay with a PayPal account or Venmo.

    PayPal Checkout
    3.49% + 49¢
    Braintree cards
    2.89% + 29¢
    Venmo
    3.49% + 49¢
  5. 05

    Adyen

    Large businesses that can meet a minimum invoice set by industry.

    Visa, Mastercard
    $0.13 + interchange++ + 0.60%
    Minimum invoice
    by industry, not published
  6. 06

    Helcim

    Businesses that want interchange-plus card pricing.

    Online, up to $50K/mo
    interchange + 0.50% + 25¢
    Recurring payments
    +0.4%
    Monthly
    $0

02Payment or commerce

Do you need a payment API or a commerce API?

A payment API moves money. If you sell products or services, you also need an order record, prices by variant, stock that can’t oversell, discount codes, sales tax, shipping, and refunds that know which item came back. Stripe’s fulfillment guide has you write a fulfillment function that handles repeat calls and records fulfillment status, and leaves saving line items in your own database and updating inventory or stock records to your code.

Flint keeps Stripe’s processing and makes the order the record. Your code sends a cart by variant, Flint prices it, applies promotions, computes US sales tax, holds the stock, and takes the payment, and one order.paid event tells you to fulfill. Flint’s card rate, 3.79% + 35¢, is higher than a processor’s alone because it covers Stripe’s fees and that commerce system, with no monthly fee.

Every checkout, payment link, invoice, and renewal creates an order with line items and server-computed totals.

POST /v1/orders

Products carry options and variants, and stock is held per location when payment starts.

held_quantity

Codes and automatic promotions stack by rules you set, with a preview call first.

combines_with

Automatic US sales tax is computed after discounts and recorded on each line.

tax_money

Refund one line item, and its share of the discount and the tax goes back with it.

line_item_allocations

Flint vs Stripe, need by need →Stripe alternatives, ranked →Does Stripe have an Orders API? →

03Developer experience

Which payment API is easiest to build on with a coding agent?

Everything below is public before you sign up: the spec, the docs server, the reference file, and the guides. A coding agent can read all of it, and the CLI covers the local loop.

Terminal: account, local webhooks, docs for your agent
# Install the CLI and create an account. The first sandbox key
# goes straight into your OS keychain.
npm install -g @flintpay/cli
flint signup

# Forward your sandbox's webhook events to a local handler,
# signed with a per-session secret. No tunnel.
flint listen --forward-to http://localhost:8080/webhooks/flint

# Give your coding agent the docs, read-only, with no key.
claude mcp add --transport http flint-docs \
  https://developers.withflintpay.com/mcp

flint listen signs each forwarded event, so the handler you test locally is the one you ship.

Node: verify a delivery and fulfill
import { Client } from "@flintpay/node";

const flint = new Client({
  baseUrl: "https://api.withflintpay.com",
  apiKey: process.env.FLINT_API_KEY!,
});

// Checks the Standard Webhooks headers (webhook-id,
// webhook-timestamp, webhook-signature) with a 300 second
// tolerance, and throws on a mismatch.
const { event, known } = flint.verifyWebhook(rawBody, headers, [
  process.env.FLINT_WEBHOOK_SECRET!,
]);

if (known && event.event_type === "order.paid") {
  // Fulfill: the order carries its lines, tax, and payments.
}

The PHP SDK has the same verifyWebhook method. Any Standard Webhooks library works too.

Every operation in a public OpenAPI spec, which you can read without an API key.

api.withflintpay.com/v1/openapi.json

A hosted docs MCP server for Claude Code, Cursor, and other agents. Read-only, and it needs no key.

developers.withflintpay.com/mcp

SKILL.md holds the HTTP API reference in one file, llms.txt lists the docs for agents, and any docs page loads as Markdown with .md on its URL.

developers.withflintpay.com/SKILL.md

A CLI whose commands map to documented routes, forwards sandbox webhooks to localhost, and serves its commands to agents as MCP tools.

flint listen

Node and PHP SDKs for server-side code, each with webhook verification that takes a list of secrets for rotation.

@flintpay/node

Sandboxes with their own orders, customers, products, and webhook endpoints, created and reset by API.

POST /v1/developer/sandboxes

Webhooks signed in the Standard Webhooks format, with up to 9 delivery attempts over about three days.

webhook-signature

Dated API versions: at most two breaking releases a year, each supported for at least 24 months after its successor ships, with a 72-hour rollback.

Flint-Version

Idempotency keys on writes: a retry within 24 hours returns the first response instead of running again.

Idempotency-Key

Flint for developers →MCP servers →AI agents guide →CLI guide →SDKs →

04Who Flint is for

Flint is built for US businesses selling online.

If you are building for one, it is the first payment API to try. Everything above is running the day you create your account.

US businesses

Flint onboards businesses in the US and pays out in USD to your own bank account. Buyers can pay from anywhere with a card.

Online payments

Hosted checkout, payment links, invoices, or Stripe Elements on your own web pages, with cards, Apple Pay, Google Pay, ACH debit, and Affirm.

Server-side SDKs

Node and PHP SDKs for your backend, or plain HTTP and JSON from any language.

Developers and agents

A public OpenAPI spec, a CLI, llms.txt, SKILL.md, and MCP servers a coding agent can read.

05Switching

How do you try Flint as your payment API?

In a sandbox, from a terminal, before you change anything in production.

  1. Create an account and a sandbox

    POST /v1/developer/sandboxes

    Run flint signup, or create an account and sandboxes in the dashboard. Each sandbox has its own data and test keys.

  2. Model the catalog

    POST /v1/products

    Create products with options and variants. Stock is tracked per location.

  3. Send a cart as an order

    POST /v1/orders

    Create an order from variant IDs and quantities. Flint prices each line, applies promotions, and computes US sales tax.

  4. Take the payment

    POST /v1/checkout-sessions

    Open a hosted checkout for the order, or confirm with Stripe Elements on your own page, and pay with Stripe's test cards.

  5. Fulfill from order.paid

    order.paid

    Run flint listen, handle order.paid locally with the same verification you ship, then switch to live keys.

Accept your first payment →

06Start

From a sandbox key to a paid test order in ten minutes.

Node

npm install @flintpay/node

CLI

npm install -g @flintpay/cli

Test keys are prefixed flint_test_ and bound to a sandbox with its own data, so nothing you try there touches live money. Pay with 4242 4242 4242 4242, any future expiry, any CVC.

Then read these

  1. 01
    The payment APIPayments with the order, tax, and stock attached
  2. 02
    Flint vs Stripe
  3. 03
    Stripe alternatives
  4. 04
    Accept your first payment
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Sources

Where each claim comes from.

  • Domestic cards 2.9% + 30¢; Stripe Tax Basic $0.50 per API transaction or 0.5% no-code; Billing 0.7% of Billing volume; no monthly fees on standard pricing. Stripe: Pricing

    stripe.com · checked 2026-10-15

  • The fulfill_checkout function must handle repeat and concurrent calls, retrieve the session with line items, check payment_status, perform fulfillment, and record fulfillment status. Saving payment details and line items in your own database and updating inventory or stock records are listed as things you might want it to do. Stripe: Fulfill orders

    docs.stripe.com · checked 2026-10-15

  • Online and Checkout API 2.9% + 30¢; card not present 3.5% + 15¢; Orders API with a non-Square processor 1% per transaction. Square: Payments pricing

    developer.squareup.com · checked 2026-10-15

  • PayPal Checkout 3.49% + 49¢; standard cards 2.99% + 49¢; advanced cards 2.89% + 29¢; last updated 2026-10-01. PayPal: Merchant fees

    www.paypal.com · checked 2026-10-15

  • Cards and digital wallets 2.89% + 29¢; Venmo 3.49% + 49¢; last updated 2026-05-07. PayPal Braintree fees

    www.paypal.com · checked 2026-10-15

  • Visa and Mastercard $0.13 + interchange++ + 0.60%; no monthly fees; a minimum invoice by industry or business model. Adyen: Pricing

    www.adyen.com · checked 2026-10-15

  • Keyed and online interchange + 0.50% + 25¢ up to $50K a month; recurring +0.4%; no monthly fees. Helcim: Pricing

    www.helcim.com · checked 2026-10-15

last reviewed 2026-10-15 · corrections: Flint Help

FAQ

Questions people ask.

What is the best payment API for developers?

For a developer building a US website or app that sells products or services, Flint Pay. It keeps Stripe's card processing and adds the commerce system in the same API: orders, variants, inventory held at payment, promotions, automatic US sales tax, shipping, returns, invoices, and subscriptions. Its OpenAPI spec is public with no key, its docs have an MCP server and SKILL.md for coding agents, and its CLI forwards webhooks to localhost.

What is the best payment API for a website?

For a US business selling on its own website, Flint Pay. Send the cart as an order and open a hosted checkout, or put a payment link behind any button. Flint prices the order, computes US sales tax, holds the stock, and processes the card on Stripe, at 3.79% + 35¢ per card payment with no monthly fee.

What is the best payment gateway API for a small business?

Flint Pay, for a small US business selling online. There is no monthly fee, no per-seat or per-API-call fee, and one card rate, 3.79% + 35¢, which covers Stripe’s processing fees and includes checkout, payment links, orders, catalog and inventory, discounts, returns, refunds, and the buyer account. Invoices and subscriptions are 0.40% each, and automatic tax is 0.50% on payments that carry tax. Flint pricing

Is Flint a good alternative to Stripe for developers?

Yes, if you are building a US website or app that sells something and would otherwise write the order, stock, promotion, tax, and refund logic around Stripe yourself. Stripe still processes every card on Flint, and card details go from the buyer’s browser to Stripe. Flint serves US businesses in USD. Create an account from the terminal with flint signup. Flint vs Stripe

Does Flint work with AI coding agents?

Yes. The hosted docs MCP server at developers.withflintpay.com/mcp is read-only and needs no key, SKILL.md holds the HTTP API reference in one file, llms.txt lists the docs, and the OpenAPI spec is public. To let an agent act on your account, flint mcp serve exposes the CLI’s commands as MCP tools, with confirmation gates for live mode. Flint MCP servers

Which SDKs does Flint have?

Node (@flintpay/node) and PHP (flintpay/flint), both for server-side code, plus a CLI. The API is plain HTTP and JSON, so any language can call it, and the OpenAPI spec is public for generating your own client.

Payments and the commerce system, in one API.

Free sandbox, no credit card, no sales call. Run flint signup, create an order by variant, and watch order.paid arrive on localhost.

Sign up
Accept your first payment

Or run npm install -g @flintpay/cli && flint signup to create your account and a sandbox key from the terminal. CLI guide

More comparisons · last reviewed 2026-10-15