Spree Shipping

The shipping system answers two questions: where is this order going (the shipping method) and where is it shipping from (the stock location). Together they produce one or more Shipments — each Shipment represents a package leaving a specific StockLocation via a specific ShippingMethod.

The shipping graph

Order
  └── Shipment (one per fulfillment package — may be many per order)
        ├── StockLocation        — where it ships from
        ├── ShippingRate × n     — offered rates from configured methods
        │    └── ShippingMethod  — UPS Ground, USPS Priority, etc.
        └── InventoryUnit × n    — per LineItem, each carrying a quantity (may split, e.g. partial backorder)

ShippingMethod
  ├── ShippingCategory × n       — which categories of items this method handles
  ├── Calculator                 — how much (per-order, per-item, weight-based)
  └── Zone × n                   — where this method applies