Create the traveler's account from their guest checkout session and sign them in. The one-time token comes from the magic link emailed during guest checkout. Re-posting the token after a successful exchange returns the already-created signed-in state without creating a second account. When the selection hold lapsed before the exchange, the account is still created and session.selectionExpired is true. bizawayToken carries the guest's BizAway session token for the card form on Payment. When it cannot be minted the exchange fails with 503 BIZAWAY_SESSION_UNAVAILABLE and returns no accessToken; re-posting the same token retries until the link expires.