post
https://apis.kentro.live/store/v1/invoices.payLink
Creates (or reuses) the hosted payment page for the open balance of an invoice (id), for a new card; send the buyer to redirectUrl. When the buyer comes back, read invoices.paymentStatus.
Who may call it: A signed-in buyer (X-Storefront-Buyer) whose account is confirmed (session.get answers confirmed: true); an unconfirmed buyer is refused with 403 CONFIRMATION_REQUIRED.
