Errors
billing_checkout_preparing
A paid checkout is still being provisioned and is safe to poll.
billing_checkout_preparing is the compatibility error returned to older clients while a paid checkout is still being provisioned. New clients should send X-Everyn-Checkout-Readiness: 202 and treat the equivalent HTTP 202 response as a non-error preparing state.
Recovery guidance
Wait for the server-provided retryAfterMilliseconds value, then repeat the exact request with the same idempotency key and request body. Do not create a new checkout intent while this request is preparing. Preserve the response requestId if the state does not progress.