Royal Mail Shipping API V3 (REST, UK)

Royal Mail Shipping API V3 is the REST surface for shipments originating in the UK on Royal Mail's network. It covers shipment creation, label retrieval, manifest closing, and tracking-event lookup. Authentication is a static bearer API token issued through the Royal Mail Customer Portal — not OAuth. The token rides on every request in an `Authorization: Bearer <token>` header and must be treated as a long-lived secret. The most common first-time integration confusion is treating the API as a programmatic mirror of the Click & Drop web UI. They are related but not identical: Click & Drop sessions and Shipping API V3 tokens are separate credentials, and some operations available in Click & Drop (like complex multi-leg returns) require additional Royal Mail-side configuration before they are available through the API. Operationally, Shipping API V3 returns label artifacts as base64-encoded payloads embedded in the JSON response. Label-format negotiation (PDF, ZPL, EPL) happens via request parameters, and printer drift between sandbox-style proofs and production printers is a recurring incident category. Manifesting is explicit: shipments must be added to a manifest and the manifest closed before the carrier handover, and missed manifest closes are a frequent cause of unexpected shipment-status drift downstream.