> ## Documentation Index > Fetch the complete documentation index at: https://docs.arcuserp.com/llms.txt > Use this file to discover all available pages before exploring further. # Purchase a package shipping label > Purchases a carrier shipping label for a package using a previously fetched rate, saving the label URL and tracking number to the package. Returns 409 with code `order_not_spendable` if the package's order is cancelled, archived, or expired, evaluated immediately before the carrier is called; idempotent via the `Idempotency-Key` header. ## OpenAPI ````yaml /openapi.yaml post /packages/{id}/buy-label openapi: 3.1.0 info: title: Arcus ERP Public API version: 1.0.0 description: > Arcus ERP public REST API. Designed for external integrations and data migration. **Authentication.** Bearer token (API key) via the `Authorization` header. Format: `Authorization: Bearer ark_live_ent__` (or `ark_test_*` for sandbox). API keys are issued per-entity in **Settings > Developers > API Keys**. **Entity scoping.** The entity is encoded in the API key prefix; routes are flat (e.g. `/v1/accounts`, `/v1/orders`, `/v1/products`). A small set of platform endpoints (migration, reconciliation, events, webhook endpoints, API keys) use the `/v1/entities/{entity_id}/...` form -- those are noted in their tags. **Key capabilities.** - Related-resource hydration via `?expand[]=` (see `x-arcus-expand` on each resource). - Cursor-based pagination (`starting_after` / `ending_before` / `limit`). - Idempotency via the `Idempotency-Key` header. - Webhook events for asynchronous notification. - Conditional requests / ETag for cache validation. **Changelog.** Entries are dated and name every published contract whose MEANING moved, not only the ones whose field names changed. The narrative version of the same entries, written for integrators, is published at https://arcuserp.mintlify.app/changelog. **2026-09-22 (planned 2026-09-21), REORDER-BUYER-TRUTH: demand changed what it MEANS on three published contracts, with no field renamed.** An integrator that pins field names sees no breakage and different numbers, which is why this entry exists. - **Demand now counts build consumption.** `demand_avg_per_day` on the public product, kit and inventory-balance objects, and `daily_demand` / `demand_basis` / `net_suggested_qty` on `GET /v1/purchasing/reorder-report`, are composed from fulfilled sales lines PLUS posted `build_consume` inventory draws: each physical decrement of a product counts exactly once. The previous rule adopted an internal-consumption basis only when the sales blend was exactly zero, so a product both sold AND consumed into work orders planned as if the build draws did not exist. Products drawn into work orders move; on one production-shaped dataset five did, the largest from 0.05/day to 14.95/day. - **`demand_basis` now carries four values, not two:** `sales`, `sales_and_builds`, `builds` and `consumption`. A consumer with a two-branch reader (anything that is not `consumption` is `sales`) silently hides the two new ones. - **`current_demand_units` on `GET /v1/inventory/balances/:id` moved, by a second rule.** It counts committed-but-unshipped CUSTOMER demand, and it now counts a kit component's own line rather than its parent kit line, and excludes non-sales documents. On one production dataset 269 of 1,043 balance rows changed, 227 of them downward; the largest single move was 1,941 to 25, on a product whose open PURCHASE order line had been reported as customer demand. Re-baseline anything that alerts or reorders off this field. - `run_rate_30/90/180/365` and `blended_daily` on the same balance object move for the first reason above. - **Added, not changed:** `GET /v1/purchasing/reorder-report` accepts `demand_window` and returns the unit, cover-through and reorder-point-provenance fields documented on that operation; `order_multiple` is accepted and returned on the product-vendor doors. - **Volume note.** The `inventory.low_stock`, `inventory.out_of_stock` and `inventory.back_in_stock` webhook events are population-gated on `on_hand <= reorder_point`, and reorder points move with the demand above, so subscribers should expect a one-time step change in event volume around the release. servers: - url: https://api.arcuserp.com/v1 description: Arcus ERP API (accepts both live `ark_live_*` and test `ark_test_*` keys) - url: https://dev-api.arcuserp.com/v1 description: >- Dev sandbox API (test-only data, accepts `ark_test_*` keys against dev RDS) security: [] paths: /packages/{id}/buy-label: parameters: - name: id in: path required: true schema: type: string format: uuid post: tags: - Fulfillment summary: Purchase a package shipping label description: >- Purchases a carrier shipping label for a package using a previously fetched rate, saving the label URL and tracking number to the package. Returns 409 with code `order_not_spendable` if the package's order is cancelled, archived, or expired, evaluated immediately before the carrier is called; idempotent via the `Idempotency-Key` header. operationId: buyPackageLabel parameters: - $ref: '#/components/parameters/IdempotencyKey' requestBody: content: application/json: schema: type: object properties: rate_object_id: type: string carrier: type: string service_token: type: string service_name: type: string shipment_object_id: type: string parcel_object_id: type: string estimated_amount: type: number label_file_type: type: string enum: - PDF - ZPLII - PNG default: PDF responses: '200': description: > Purchased label record. NEW-GAP-RERATE-SHIPPING-CHARGE-DELTA-WORKFLOW (2026-05-16): the response now includes a `shipping_cost_analysis` block when the carrier cost diverges from what the customer was charged for shipping. When `suggest_action !== 'absorb'`, the operator (or a downstream automation) should call `POST /packages/{id}/recharge-label` to settle the delta. content: application/json: schema: allOf: - $ref: '#/components/schemas/PackageLabel' - type: object properties: shipping_cost_analysis: nullable: true type: object description: | Carrier-cost-vs-customer-charged delta. Null when the analysis could not be computed (logged server-side as a non-blocking warning). properties: customer_charged: type: number description: orders.shipping_total at time of label purchase carrier_cost: type: number description: just-purchased label actual_amount delta: type: number description: | carrier_cost - customer_charged. Positive means Arcus is absorbing cost (upgrade case); negative means customer overpaid. suggest_action: type: string enum: - recharge - refund_overpay - absorb description: | recharge -- delta > threshold; bill the customer refund_overpay -- delta < -threshold; refund absorb -- |delta| <= threshold; no operator action threshold: type: number description: per-entity setting, default $2 label_id: type: string format: uuid nullable: true package_id: type: string format: uuid nullable: true order_id: type: string format: uuid nullable: true '400': $ref: '#/components/responses/Error' '401': $ref: '#/components/responses/Error' '403': $ref: '#/components/responses/Error' '404': $ref: '#/components/responses/Error' '409': description: | `order_not_spendable` -- the package's order is terminal (cancelled, archived, or expired) and cannot buy carrier labels. Body carries `order_status` and `order_number` alongside the standard error envelope. CANCEL-PACKAGE-LIFECYCLE WS-A, 2026-07-27. content: application/json: schema: allOf: - $ref: '#/components/schemas/Error' - type: object properties: order_status: type: string enum: - cancelled - archived - expired order_number: type: string nullable: true '422': $ref: '#/components/responses/Error' security: - ApiKeyAuth: [] components: parameters: IdempotencyKey: name: Idempotency-Key in: header required: false description: | Client-generated unique key for idempotent POST/PATCH/DELETE operations. Alias for the Idempotency parameter. Max 255 chars. On retry with the same key, the original response is returned without re-executing the operation. Keys expire after 24 hours. schema: type: string maxLength: 255 schemas: PackageLabel: type: object description: > A shipping label purchased from a carrier (typically Shippo). One label per package. URLs expire 30 days after purchase; persist the PDF locally for long-term retention. properties: id: type: string format: uuid package_id: type: string format: uuid carrier: type: string service_level: type: string tracking_number: type: string label_url: type: string description: Carrier-hosted PDF URL (time-limited). label_pdf_url: type: string nullable: true description: Arcus-archived PDF URL (permanent). purchased_cost: type: number description: Amount Arcus paid the carrier. billed_cost: type: number nullable: true description: Amount billed to the customer. purchased_at: type: string format: date-time readOnly: true voided_at: type: string format: date-time nullable: true readOnly: true Error: description: | Alias for ErrorEnvelope. Canonical error response shape used by all API endpoints. Refer to ErrorEnvelope for the full field definition. allOf: - $ref: '#/components/schemas/ErrorEnvelope' ErrorEnvelope: type: object description: | Canonical error response envelope. All API errors use this shape. required: - error - code properties: error: type: string description: Machine-readable error key example: not_found code: type: string description: Machine-readable error code (often same as error) example: not_found type: type: string enum: - validation_error - permission_error - not_found - conflict - rate_limit - internal - expand_error - not_implemented example: not_found hint: type: string description: Human-readable one-sentence explanation (English) example: >- The requested order does not exist or does not belong to this entity. param: type: string description: The parameter that caused the error, if applicable example: expand[0] required: type: string description: The scope required (only on insufficient_scope errors) example: accounts:read request_id: type: string description: >- Unique request ID for support tracing (maps to CloudWatch log stream) example: req_abc123 responses: Error: description: Error response (400/401/403/404/409/422/429/500) content: application/json: schema: $ref: '#/components/schemas/ErrorEnvelope' securitySchemes: ApiKeyAuth: type: http scheme: bearer description: | API key issued per entity via Settings > Developers > API Keys. Each key carries scopes (e.g. orders:read, products:write). Bearer token format: Authorization: Bearer ark_live_ent__ Test keys use ark_test_ent__. Both are issued per entity via Settings > Developers > API Keys. ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.