> ## Documentation Index
> Fetch the complete documentation index at: https://docs.billions.works/llms.txt
> Use this file to discover all available pages before exploring further.

# Introduction

> Accept payments with Billions in your application or online store.

Billions provides a hosted payment flow for accepting payments without building
your own checkout or payment-status infrastructure.

Your backend creates an invoice, redirects the customer to the hosted payment
page, and confirms the result by retrieving the invoice from the authenticated
Billions API.

<CardGroup cols={2}>
  <Card title="Integrate the payment flow" icon="diagram-project" href="/integration/overview">
    Follow the complete path from API credentials to production launch.
  </Card>

  <Card title="Start from an example" icon="code" href="/examples/create-an-invoice">
    Create an invoice, handle a callback, and retrieve invoices from the API.
  </Card>

  <Card title="Choose an SDK" icon="box" href="/sdks/overview">
    Use the TypeScript or PHP SDK to integrate the Billions API.
  </Card>

  <Card title="Connect a store" icon="store" href="/store-integrations/overview">
    Add Billions to WooCommerce with the official plugin.
  </Card>
</CardGroup>

## How the integration works

<Steps>
  <Step title="Create an invoice">
    Your backend sends the amount, currency, order reference, callback URL, and
    an idempotency key to Billions.
  </Step>

  <Step title="Redirect the customer">
    Open the returned `invoice_url`. The customer completes payment on the
    Billions-hosted page.
  </Step>

  <Step title="Receive a callback">
    Billions notifies your callback endpoint that the invoice may have changed.
  </Step>

  <Step title="Verify with the API">
    Retrieve the invoice from the authenticated API and update the order only
    from that response.
  </Step>
</Steps>

<Warning>
  Never mark an order as paid from callback fields alone. Treat the callback as
  a signal to retrieve the current invoice from the Billions API.
</Warning>

## Choose a path

* Building a custom application? Start with the [integration guide](/integration/overview).
* Using Node.js or another TypeScript runtime? See the [TypeScript SDK](/sdks/typescript).
* Building with PHP 8.1 or newer? See the [PHP SDK](/sdks/php).
* Running WooCommerce? Install the [WooCommerce integration](/store-integrations/woocommerce).