Create an EasyPost Batch of up to 1,000 shipments, trigger asynchronous label purchase, and retrieve the combined label PDF

Steps

1. POST to /v2/batches with a 'shipments' array; each element must include from_address, to_address, parcel, carrier, service, and a single-element carrier_accounts array
2. The batch is created in 'creation_queued' state; poll GET /v2/batches/{id} or use a webhook to detect when state transitions to 'created' before issuing the buy call
3. POST to /v2/batches/{id}/buy to enqueue asynchronous label purchase; monitor the batch state via webhook or polling until it reaches 'purchased'
4. Once purchased, POST to /v2/batches/{id}/label with 'file_format': 'PDF' (or 'ZPLII') to generate a merged label file; poll until the label_url is populated
5. Keep each batch under 1,000 shipments; for larger volumes, split into multiple batches and process concurrently