Strategy for testing APIs that call third-party services — mock, contract, or live?

Our payments API calls Stripe, our notifications API calls SendGrid, and our geolocation API calls Google Maps. All have sandbox environments. For each external dependency I'm unsure whether to: 1. **Mock at the HTTP level** — fastest, but drifts from reality 2. **Use sandbox/test modes** — real integration, but flaky, slow, costs money 3. **Contract tests** — validates the interface without calling live services Is the answer just "it depends" or is there a principled framework for deciding? Also curious how you handle the case where the sandbox doesn't behave identically to production.