> ## Documentation Index
> Fetch the complete documentation index at: https://docs.grain.inc/llms.txt
> Use this file to discover all available pages before exploring further.
# Installation & Setup
> Set up the Payment SDK in your application
## Prerequisites
* A Grain merchant account with a **Merchant ID**
* A server-side **API key** for creating payment sessions
* Node.js 18+ (for Next.js applications)
## Get Your Credentials
Log in to the [Grain Dashboard](https://dashboard.grain.inc) and navigate to **Settings > API Keys**. Copy your **Merchant ID** — this is used to initialize the client-side SDK.
From the same page, copy your **API Key**. This is your server-side secret used to create payment sessions.
Never expose your API key in client-side code. It should only be used in server-side route handlers or backend services.
## Configure Environment Variables
Add the following to your `.env` file:
```bash theme={null}
# Required
NEXT_PUBLIC_CUBEPAY_MERCHANT_ID=your-merchant-id # Client-side: SDK initialization
CUBEPAY_API_KEY=your-api-key # Server-side: session creation
# Optional
CUBEPAY_API_HOST=https://api.pay.grain.inc # API host (defaults to production)
```
## Add the SDK Mount Point
The Payment SDK renders its payment modal inside a dedicated DOM element. Add this to your root layout:
```tsx theme={null}
// app/layout.tsx
export default function RootLayout({ children }: { children: React.ReactNode }) {
return (
{children}
);
}
```
The SDK creates a **Shadow DOM** inside this element, so its styles are fully isolated from your application. No CSS conflicts.
## Load the SDK Bundle
The client-side SDK is distributed as a UMD bundle. Place it in your public directory:
```
public/
vendor/
cubist-pay-client-sdk.umd.js
```
The SDK service loads this script lazily on first use — no manual script tags needed.
## Environment Comparison
| Feature | Sandbox | Production |
| ------------------- | --------------------------- | ------------------------- |
| Blockchain networks | Testnets (Sepolia) | Mainnets (Ethereum, Base) |
| Real funds | No | Yes |
| Webhooks | Supported | Supported |
| Settlement | Simulated | Real on-chain settlement |
| API host | `api.sandbox.pay.grain.inc` | `api.pay.grain.inc` |
## Next Steps
Proceed to [Initialize the SDK](/payment-sdk/integration-guide/initialize-sdk) to set up the SDK service and process your first payment.