Requests one or more mail items to be shipped to the given address and recipient. Mail items in a single call may be split into multiple shipments; each shipment produces one entry in the response. If any item is in a state that cannot be shipped (already shipped, shredded, deposited, or past its storage deadline), the entire request is rejected with a 409 and no shipments are created. Shipping cost is determined asynchronously after labels are priced; the final cost arrives via the mailItem.forward.completed webhook and on GET /v1/mail-items/{id}. When consuming cost from the webhook, deduplicate via forwardDetails.id, which names the same shipment under an older name: mail items in the same shipment share the cost.