paypal-integration - Agent Skills

PayPal Integration

Master PayPal payment integration including Express Checkout, IPN handling, recurring billing, and refund workflows.

Do not use this skill when

The task is unrelated to paypal integration

You need a different domain or tool outside this scope

Instructions

Clarify goals, constraints, and required inputs.

Apply relevant best practices and validate outcomes.

Provide actionable steps and verification.

If detailed examples are required, open resources/implementation-playbook.md.

Use this skill when

Integrating PayPal as a payment option

Implementing express checkout flows

Setting up recurring billing with PayPal

Processing refunds and payment disputes

Handling PayPal webhooks (IPN)

Supporting international payments

Implementing PayPal subscriptions

Core Concepts

1. Payment Products

PayPal Checkout

One-time payments

Express checkout experience

Guest and PayPal account payments

PayPal Subscriptions

Recurring billing

Subscription plans

Automatic renewals

PayPal Payouts

Send money to multiple recipients

Marketplace and platform payments

2. Integration Methods

Client-Side (JavaScript SDK)

Smart Payment Buttons

Hosted payment flow

Minimal backend code

Server-Side (REST API)

Full control over payment flow

Custom checkout UI

Advanced features

3. IPN (Instant Payment Notification)

Webhook-like payment notifications

Asynchronous payment updates

Verification required

Quick Start

// Frontend - PayPal Smart Buttons
<div id="paypal-button-container"></div>
<script src="https://www.paypal.com/sdk/js?client-id=YOUR_CLIENT_ID&currency=USD"></script>
<script>
  paypal.Buttons({
    createOrder: function(data, actions) {
      return actions.order.create({
        purchase_units: [{
          amount: {
            value: '25.00'
          }
        }]
      });
    },
    onApprove: function(data, actions) {
      return actions.order.capture().then(function(details) {
        // Payment successful
        console.log('Transaction completed by ' + details.payer.name.given_name);        // Send to backend for verification
        fetch('/api/paypal/capture', {
          method: 'POST',
          headers: {'Content-Type': 'application/json'},
          body: JSON.stringify({orderID: data.orderID})
        });
      });
    }
  }).render('#paypal-button-container');
</script>

# Backend - Verify and capture order
from paypalrestsdk import Payment
import paypalrestsdk
paypalrestsdk.configure({
    "mode": "sandbox",  # or "live"
    "client_id": "YOUR_CLIENT_ID",
    "client_secret": "YOUR_CLIENT_SECRET"
})
def capture_paypal_order(order_id):
    """Capture a PayPal order."""
    payment = Payment.find(order_id)    if payment.execute({"payer_id": payment.payer.payer_info.payer_id}):
        # Payment successful
        return {
            'status': 'success',
            'transaction_id': payment.id,
            'amount': payment.transactions[0].amount.total
        }
    else:
        # Payment failed
        return {
            'status': 'failed',
            'error': payment.error
        }

Express Checkout Implementation

Server-Side Order Creation

import requests
import json
class PayPalClient:
    def __init__(self, client_id, client_secret, mode='sandbox'):
        self.client_id = client_id
        self.client_secret = client_secret
        self.base_url = 'https://api-m.sandbox.paypal.com' if mode == 'sandbox' else 'https://api-m.paypal.com'
        self.access_token = self.get_access_token()
    def get_access_token(self):
        """Get OAuth access token."""
        url = f"{self.base_url}/v1/oauth2/token"
        headers = {"Accept": "application/json", "Accept-Language": "en_US"}
        response = requests.post(
            url,
            headers=headers,
            data={"grant_type": "client_credentials"},
            auth=(self.client_id, self.client_secret)
        )
        return response.json()['access_token']
    def create_order(self, amount, currency='USD'):
        """Create a PayPal order."""
        url = f"{self.base_url}/v2/checkout/orders"
        headers = {
            "Content-Type": "application/json",
            "Authorization": f"Bearer {self.access_token}"
        }
        payload = {
            "intent": "CAPTURE",
            "purchase_units": [{
                "amount": {
                    "currency_code": currency,
                    "value": str(amount)
                }
            }]
        }
        response = requests.post(url, headers=headers, json=payload)
        return response.json()
    def capture_order(self, order_id):
        """Capture payment for an order."""
        url = f"{self.base_url}/v2/checkout/orders/{order_id}/capture"
        headers = {
            "Content-Type": "application/json",
            "Authorization": f"Bearer {self.access_token}"
        }
        response = requests.post(url, headers=headers)
        return response.json()
    def get_order_details(self, order_id):
        """Get order details."""
        url = f"{self.base_url}/v2/checkout/orders/{order_id}"
        headers = {
            "Authorization": f"Bearer {self.access_token}"
        }        response = requests.get(url, headers=headers)
        return response.json()

IPN (Instant Payment Notification) Handling

IPN Verification and Processing

from flask import Flask, request
import requests
from urllib.parse import parse_qs
app = Flask(__name__)
@app.route('/ipn', methods=['POST'])
def handle_ipn():
    """Handle PayPal IPN notifications."""
    # Get IPN message
    ipn_data = request.form.to_dict()
    # Verify IPN with PayPal
    if not verify_ipn(ipn_data):
        return 'IPN verification failed', 400
    # Process IPN based on transaction type
    payment_status = ipn_data.get('payment_status')
    txn_type = ipn_data.get('txn_type')
    if payment_status == 'Completed':
        handle_payment_completed(ipn_data)
    elif payment_status == 'Refunded':
        handle_refund(ipn_data)
    elif payment_status == 'Reversed':
        handle_chargeback(ipn_data)
    return 'IPN processed', 200
def verify_ipn(ipn_data):
    """Verify IPN message authenticity."""
    # Add 'cmd' parameter
    verify_data = ipn_data.copy()
    verify_data['cmd'] = '_notify-validate'
    # Send back to PayPal for verification
    paypal_url = 'https://ipnpb.sandbox.paypal.com/cgi-bin/webscr'  # or production URL
    response = requests.post(paypal_url, data=verify_data)
    return response.text == 'VERIFIED'
def handle_payment_completed(ipn_data):
    """Process completed payment."""
    txn_id = ipn_data.get('txn_id')
    payer_email = ipn_data.get('payer_email')
    mc_gross = ipn_data.get('mc_gross')
    item_name = ipn_data.get('item_name')
    # Check if already processed (prevent duplicates)
    if is_transaction_processed(txn_id):
        return
    # Update database
    # Send confirmation email
    # Fulfill order
    print(f"Payment completed: {txn_id}, Amount: ${mc_gross}")
def handle_refund(ipn_data):
    """Handle refund."""
    parent_txn_id = ipn_data.get('parent_txn_id')
    mc_gross = ipn_data.get('mc_gross')
    # Process refund in your system
    print(f"Refund processed: {parent_txn_id}, Amount: ${mc_gross}")
def handle_chargeback(ipn_data):
    """Handle payment reversal/chargeback."""
    txn_id = ipn_data.get('txn_id')
    reason_code = ipn_data.get('reason_code')    # Handle chargeback
    print(f"Chargeback: {txn_id}, Reason: {reason_code}")

Subscription/Recurring Billing

Create Subscription Plan

def create_subscription_plan(name, amount, interval='MONTH'):
    """Create a subscription plan."""
    client = PayPalClient(CLIENT_ID, CLIENT_SECRET)
    url = f"{client.base_url}/v1/billing/plans"
    headers = {
        "Content-Type": "application/json",
        "Authorization": f"Bearer {client.access_token}"
    }
    payload = {
        "product_id": "PRODUCT_ID",  # Create product first
        "name": name,
        "billing_cycles": [{
            "frequency": {
                "interval_unit": interval,
                "interval_count": 1
            },
            "tenure_type": "REGULAR",
            "sequence": 1,
            "total_cycles": 0,  # Infinite
            "pricing_scheme": {
                "fixed_price": {
                    "value": str(amount),
                    "currency_code": "USD"
                }
            }
        }],
        "payment_preferences": {
            "auto_bill_outstanding": True,
            "setup_fee": {
                "value": "0",
                "currency_code": "USD"
            },
            "setup_fee_failure_action": "CONTINUE",
            "payment_failure_threshold": 3
        }
    }
    response = requests.post(url, headers=headers, json=payload)
    return response.json()
def create_subscription(plan_id, subscriber_email):
    """Create a subscription for a customer."""
    client = PayPalClient(CLIENT_ID, CLIENT_SECRET)
    url = f"{client.base_url}/v1/billing/subscriptions"
    headers = {
        "Content-Type": "application/json",
        "Authorization": f"Bearer {client.access_token}"
    }
    payload = {
        "plan_id": plan_id,
        "subscriber": {
            "email_address": subscriber_email
        },
        "application_context": {
            "return_url": "https://yourdomain.com/subscription/success",
            "cancel_url": "https://yourdomain.com/subscription/cancel"
        }
    }
    response = requests.post(url, headers=headers, json=payload)
    subscription = response.json()    # Get approval URL
    for link in subscription.get('links', []):
        if link['rel'] == 'approve':
            return {
                'subscription_id': subscription['id'],
                'approval_url': link['href']
            }

Refund Workflows

def create_refund(capture_id, amount=None, note=None):
    """Create a refund for a captured payment."""
    client = PayPalClient(CLIENT_ID, CLIENT_SECRET)
    url = f"{client.base_url}/v2/payments/captures/{capture_id}/refund"
    headers = {
        "Content-Type": "application/json",
        "Authorization": f"Bearer {client.access_token}"
    }
    payload = {}
    if amount:
        payload["amount"] = {
            "value": str(amount),
            "currency_code": "USD"
        }
    if note:
        payload["note_to_payer"] = note
    response = requests.post(url, headers=headers, json=payload)
    return response.json()
def get_refund_details(refund_id):
    """Get refund details."""
    client = PayPalClient(CLIENT_ID, CLIENT_SECRET)
    url = f"{client.base_url}/v2/payments/refunds/{refund_id}"
    headers = {
        "Authorization": f"Bearer {client.access_token}"
    }    response = requests.get(url, headers=headers)
    return response.json()

Error Handling

class PayPalError(Exception):
    """Custom PayPal error."""
    pass
def handle_paypal_api_call(api_function):
    """Wrapper for PayPal API calls with error handling."""
    try:
        result = api_function()
        return result
    except requests.exceptions.RequestException as e:
        # Network error
        raise PayPalError(f"Network error: {str(e)}")
    except Exception as e:
        # Other errors
        raise PayPalError(f"PayPal API error: {str(e)}")
Usage

try:
    order = handle_paypal_api_call(lambda: client.create_order(25.00))
except PayPalError as e:
    # Handle error appropriately
    log_error(e)

Testing

# Use sandbox credentials
SANDBOX_CLIENT_ID = "..."
SANDBOX_SECRET = "..."
Test accounts

Create test buyer and seller accounts at developer.paypal.com
def test_payment_flow():
    """Test complete payment flow."""
    client = PayPalClient(SANDBOX_CLIENT_ID, SANDBOX_SECRET, mode='sandbox')
    # Create order
    order = client.create_order(10.00)
    assert 'id' in order
    # Get approval URL
    approval_url = next((link['href'] for link in order['links'] if link['rel'] == 'approve'), None)
    assert approval_url is not None    # After approval (manual step with test account)
    # Capture order
    # captured = client.capture_order(order['id'])
    # assert captured['status'] == 'COMPLETED'

Resources

references/express-checkout.md: Express Checkout implementation guide

references/ipn-handling.md: IPN verification and processing

references/refund-workflows.md: Refund handling patterns

references/billing-agreements.md: Recurring billing setup

assets/paypal-client.py: Production PayPal client

assets/ipn-processor.py: IPN webhook processor

assets/recurring-billing.py: Subscription management

Best Practices

Always Verify IPN: Never trust IPN without verification

Idempotent Processing: Handle duplicate IPN notifications

Error Handling: Implement robust error handling

Logging: Log all transactions and errors

Test Thoroughly: Use sandbox extensively

Webhook Backup: Don't rely solely on client-side callbacks

Currency Handling: Always specify currency explicitly

Common Pitfalls

Not Verifying IPN: Accepting IPN without verification

Duplicate Processing: Not checking for duplicate transactions

Wrong Environment: Mixing sandbox and production URLs/credentials

Missing Webhooks: Not handling all payment states

Hardcoded Values: Not making configurable for different environments