> ## 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.

# Integration overview

> Understand the components and responsibilities in a Billions payment integration.

A Billions integration connects four components:

1. Your checkout, where the customer starts payment.
2. Your backend, which creates and verifies invoices.
3. The Billions hosted payment page.
4. Your callback endpoint, which reacts to invoice updates.

## Responsibilities

| Component         | Responsibility                                            |
| ----------------- | --------------------------------------------------------- |
| Your frontend     | Starts checkout and redirects to `invoice_url`            |
| Your backend      | Stores credentials, creates invoices, and verifies status |
| Billions          | Hosts the payment experience and processes the payment    |
| Callback endpoint | Receives a notification and triggers API verification     |

## Recommended integration order

<Steps>
  <Step title="Prepare your account">
    Obtain an API key and decide which environment you will use.
  </Step>

  <Step title="Create invoices">
    Implement invoice creation on your backend with stable order references and
    idempotency keys.
  </Step>

  <Step title="Redirect customers">
    Send the customer to the hosted payment URL returned by the API.
  </Step>

  <Step title="Handle callbacks safely">
    Expose an HTTPS endpoint and retrieve the invoice from the API before
    changing order state.
  </Step>

  <Step title="Test failure paths">
    Cover retries, expired invoices, duplicate callbacks, and delayed payments.
  </Step>

  <Step title="Go live">
    Switch to production credentials and complete the launch checklist.
  </Step>
</Steps>

<CardGroup cols={2}>
  <Card title="Prepare the integration" icon="key" href="/integration/prerequisites">
    Gather credentials and define your order model.
  </Card>

  <Card title="Follow the payment flow" icon="arrow-right" href="/integration/payment-flow">
    Implement invoice creation through final order confirmation.
  </Card>
</CardGroup>