# Backend SDK (Payments)

This page compliments the [Basic Payment Integration](https://docs.paytweed.com/getting-started/token-checkout-or-payments/legacy-token-checkout/basic-checkout-integration) guide.\
Here you will find additional information regarding functions used in the Payments Integration Guide.

***

{% hint style="info" %}
**Note:** We show here an example for NFT checkout,\
but its the same process for Token checkout as well,\
the only difference being the name of the function and the `id` parameter.
{% endhint %}

## The \`getNftPurchaseData\` callback function[​](https://docs.paytweed.com/frontend-sdk/javascript/on-chain-functions/#buy-with-fiat) <a href="#buy-with-fiat" id="buy-with-fiat"></a>

In this function you are expected to define the transaction parameters, and define your desired settlement type.\
(For more info about settlement types see: [flow-of-funds](https://docs.paytweed.com/getting-started/token-checkout-or-payments/legacy-token-checkout/features/flow-of-funds "mention"))

For each settlement type you will have to adjust your `getNftPurchaseData` parameters accordingly:

```typescript
// Common parameters for all settlement types
const nftCheckoutCommon = {
  title: 'Tweed Demo NFT',
  fiatCurrencyId: 'USD',
  contractAddress: DEMO_NFT_CONTRACT_ADDRESS,
  chain: 'polygonAmoy',
  description: "Non-fungible token for demonstration of Tweed's capabilities",
  tokenUri: 'https://paytweed-assets.s3.amazonaws.com/pinky.png' // A simple PNG for the NFT on Tweed's widget
}

const nftCheckoutVariants: Record<string, NftPurchaseBackendPayload> = {
  'demo-freemint': {
    ...nftCheckoutCommon,
    nftId: 'demo-freemint',
    priceInCrypto: '0', // For free mint
    tokenContractAddress: undefined, // For free mint
    abi: DEMO_NFT_CONTRACT_ABI_FREE,
  },
  'demo-paidmint-native': {
    ...nftCheckoutCommon,
    nftId: 'demo-paidmint-native',
    priceInCrypto: String(1e16), // For native settlement -- set price to 0.01 MATIC
    abi: DEMO_NFT_CONTRACT_ABI_PAID_NATIVE,
  },
  'demo-paidmint-erc20': {
    ...nftCheckoutCommon,
    nftId: 'demo-paidmint-erc20',
    priceInCrypto: String(100e18), // For ERC20 settlement -- 100 fake USDC token
    tokenContractAddress: FAKE_USDC_CONTRACT_ADDRESS, // For ERC20 settlement -- fake USDC token address
    abi: DEMO_NFT_CONTRACT_ABI_PAID_ERC20,
  },
}
```

{% hint style="info" %}
*See source repository for full example:* [*https://github.com/paytweed/demo-v2-next/blob/main/services/tweed.service.ts*](https://github.com/paytweed/demo-v2-next/blob/main/services/tweed.service.ts)
{% endhint %}

Note how we provide different parameters based on the type of the settlement.

The `nftId` should correlate to the `nftId` passed from the frontend in the `buyWithFiat` function call.

#### Correctly specifiying your ABI:

The `abi` parameter should contain the name of the function and its arguments declerations,\
similar to the following example: \
`safeMint(sendTo address, tokenId uint256, tokenUri string)`. \
\
The arguments needs to get populated into the`customMintParams` object like so:

```javascript
customMintParams : {
    sendTo: <THE_WALLET_ADDRESS_VAR>,
    tokenId: <YOUR_TOKEN_ID_VAR>,
    tokenUri: <YOUR_TOKEN_URI_VAR>
}
```