Send a prepared order to checkout and return the payment link. Call only after the customer has seen the prepare-order-tool summary and explicitly confirmed it. All questions must be answered first. The customer opens checkout_url and pays themselves; billing address, VAT ID and taxes are handled by Stripe. The link is valid for 24 hours. Repeated calls return the same link while it is valid, or a new one after it expires. After this call the order can no longer be changed.