Never Commit Credentials: Do not commit Client ID or Secret to version control. Use environment variables. Protect Secret: Your Secret must never appear in frontend code or be exposed publicly. Verify IPN Messages: Always verify IPN messages with PayPal before processing payments. Use HTTPS Only: Ensure your IPN endpoint uses HTTPS with a valid SSL certificate. Secure Storage: Store PayPal credentials in WordPress options table, not in code. Regular Updates: Keep Agency Pulse Invoice and all plugins updated for security patches. Monitor Transactions: Regularly review PayPal transaction history for suspicious activity. Test Before Live: Always complete testing in Sandbox mode before enabling production payments. Account Security: Use strong passwords and enable 2FA on your PayPal account.