One-Time Purchase Integration - EVM Contract Settlement

Accept credit card payments and settle USDC directly to your EVM smart contract

This guide walks you through integrating Coinflow checkout to accept one-time credit card purchases with USDC settlement to your whitelisted EVM contract (Base, Polygon, Arbitrum, etc.).

Prerequisites

Complete these steps before starting the integration.

Create your sandbox account

Generate API keys

Create a sandbox API key for authentication

Add chargeback protection

Add the protection script to every page of your app

Whitelist your contract

Whitelist your EVM contract address for settlement.

Quick Reference

Authorization Headers

Header	Description
`Authorization`	Your API key from the merchant dashboard
`x-coinflow-auth-user-id`	Unique customer ID from your system
`x-coinflow-auth-session-key`	JWT token authorizing the payer (valid 24 hours)

Helpful Resources

Choose Your Implementation

Checkout Link

React SDK

API Only

Best for simple integrations. Generate a hosted checkout URL to redirect users or embed in an iframe.

Step 1: Generate the checkout link

$   curl -X POST https://api-sandbox.coinflow.cash/api/checkout/link \
>     -H "x-coinflow-auth-user-id: payer123" \
>     -H "Authorization: YOUR_API_KEY" \
>     -H "Content-Type: application/json" \
>     -d '{
>   "email": "payer123@gmail.com",
>   "subtotal": {
>     "cents": 500,
>     "currency": "USD"
>   },
>   "blockchain": "polygon",
>   "transactionData": {
>     "transaction": {
>       "data": "0x...",
>       "to": "0x..."
>     }
>   }
> }'

Step 2: Use the checkout link

Embed in an iframe

1 <iframe
2   allow="payment"
3   src="CHECKOUT_LINK_FROM_STEP_1"
4   style="width: 100%; height: 600px; border: none;"
5 />

Step 3: Handle success events

Listen for payment completion when using an iframe:

1 window.addEventListener('message', (event) => {
2   if (typeof event.data === 'string') {
3     const data = JSON.parse(event.data);
4     if (data.data === 'success') {
5       console.log('Payment ID:', data.info.paymentId);
6       // Handle successful payment
7     }
8   }
9 });

3DS Authentication

After implementing basic checkout, add 3DS for enhanced security. Contact Coinflow to enable 3DS on your account.

Complete Checkout with 3DS Challenge

Learn how to add 3DS to your new card and saved card requests

Chargeback Protection

Improve approval rates and reduce fraud by sharing payer events with Coinflow.

Send user events

Track key user actions throughout their journey on your app.

$ curl --request POST \
>      --url https://api-sandbox.coinflow.cash/api/events \
>      --header 'Authorization: YOUR_API_KEY' \
>      --header 'content-type: application/json' \
>      --data '{
>        "eventType": "SignUp",
>        "customerId": "user-123-abc",
>        "country": "US",
>        "username": "johndoe",
>        "email": "john@example.com",
>        "firstName": "John",
>        "lastName": "Doe"
>      }'

Required headers for checkout

When processing payments, include these headers for chargeback protection:

Header	Description
`x-device-id`	Device ID from the chargeback protection script
`x-coinflow-client-ip`	Customer’s IPv4 address
`user-agent`	Customer’s browser user agent

On sandbox, use partnerId = COINFTEST when configuring the protection script.

Next Steps

Test Your Integration

Use sandbox test cards to verify your implementation

Configure Webhooks

Receive real-time payment notifications

Go Live

Create your production merchant account

API Reference

Explore the complete API documentation

$	curl -X POST https://api-sandbox.coinflow.cash/api/checkout/link \
>	-H "x-coinflow-auth-user-id: payer123" \
>	-H "Authorization: YOUR_API_KEY" \
>	-H "Content-Type: application/json" \
>	-d '{
>	"email": "payer123@gmail.com",
>	"subtotal": {
>	"cents": 500,
>	"currency": "USD"
>	},
>	"blockchain": "polygon",
>	"transactionData": {
>	"transaction": {
>	"data": "0x...",
>	"to": "0x..."
>	}
>	}
>	}'

1	<iframe
2	allow="payment"
3	src="CHECKOUT_LINK_FROM_STEP_1"
4	style="width: 100%; height: 600px; border: none;"
5	/>

1	window.addEventListener('message', (event) => {
2	if (typeof event.data === 'string') {
3	const data = JSON.parse(event.data);
4	if (data.data === 'success') {
5	console.log('Payment ID:', data.info.paymentId);
6	// Handle successful payment
7	}
8	}
9	});

$	curl --request POST \
>	--url https://api-sandbox.coinflow.cash/api/events \
>	--header 'Authorization: YOUR_API_KEY' \
>	--header 'content-type: application/json' \
>	--data '{
>	"eventType": "SignUp",
>	"customerId": "user-123-abc",
>	"country": "US",
>	"username": "johndoe",
>	"email": "john@example.com",
>	"firstName": "John",
>	"lastName": "Doe"
>	}'