Lesson from the SOAP insurance API story (160h estimated, 700h actual): when an integration is involved Adam Piotrowski refuses fixed-price estimates. The responsibility is first on the client to verify the vendor's documentation is accurate, that others have integrated successfully, that an SLA exists, and that API errors are actionable. Practical advice: propose other work on fixed price first to build trust, then do the integration on time & materials; don't 'just follow orders' to estimate an integration you can't control, because you will be blamed at the end.