How do you fix “Invalid Form Key. Please refresh the page” at checkout?

That error means the form key the page sent doesn’t match the session. The usual causes are a full-page cache (Varnish) or CDN (Cloudflare) caching the form key, a session-cookie domain or path mismatch, or an aggressive caching rule on the checkout URL. We find which layer is serving a stale key, cache, cookie, or config, and fix the rule so genuine submissions go through without weakening CSRF protection.