> ## 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.

# Prerequisites

> Prepare credentials, endpoints, and order identifiers before coding.

Before integrating, prepare the following:

* A Billions API key stored only on your backend.
* A public HTTPS callback endpoint.
* A unique order reference in your system.
* A persistent place to store the Billions invoice ID.
* A strategy for retrying network requests safely.

<Note>
  The SDKs use `https://api.billions.works` and public API version `v1` by default.
</Note>

## Keep credentials server-side

Never embed the API key in browser code, a mobile application, a public
repository, or logs. Load it from a secret manager or environment variable.

```bash theme={null}
BILLIONS_API_KEY=your-api-key
```

## Define two identifiers

Use separate identifiers for separate jobs:

| Field            | Purpose                                     | Recommendation                               |
| ---------------- | ------------------------------------------- | -------------------------------------------- |
| `order_no`       | Connects the invoice to your business order | Use your stable internal order number        |
| `idempotencyKey` | Makes a create attempt safe to retry        | Derive it from the order and payment attempt |

An order may have more than one payment attempt. In that case, keep `order_no`
stable and use a new idempotency key for each intentional attempt.

<Warning>
  Reuse an idempotency key only when retrying the same logical invoice creation.
  Do not reuse one key for unrelated orders.
</Warning>

## Information still required

<Note>
  TODO: Add the dashboard URL, instructions for creating an API key, available
  environments, and the definitive list of supported invoice currencies.
</Note>