Create a United Kingdom shipping label via Shippo — pass a to_address, from_address and parcel (dimensions in CENTIMETRES, weight in GRAMS by default), and the server fetches carrier rates, buys the CHEAPEST rate by default (or the carrier/service you specify), and returns the shipment_id, transaction_id, tracking_number, printable label_url and the price paid (GBP). Recommended carrier: Royal Mail Tracked 24/48 via the Shippo master account — no Royal Mail contract or merchant account needed — so an agent can print a Royal Mail label with no merchant courier account. Bring your own Shippo API token via header x-shippo-token: free TEST tokens (prefix shippo_test_…) run the full flow end-to-end at no cost (sign up free, no card, at apps.goshippo.com/join); production tokens (prefix shippo_live_…) buy real labels. The token prefix auto-selects the environment. weight is always required. Owner policy headers gate the label price before any purchase.