If you're building an AI shopping/checkout agent, there's no sandbox for testing it — so I built one. acp-sandbox is a free mock merchant implementing the Agentic Commerce Protocol (ACP, backed by OpenAI/Stripe/Meta): real schema conformance for the checkout_sessions lifecycle, no signup, no real money, plus a dashboard to inspect every request/response your agent sends. Already used by a real payment-infrastructure founder who said it did exactly what he needed.