Design a Cart Pricing Calculator

Design the object model and core APIs for a checkout pricing calculator that turns a cart of line items into a final total by applying line pricing, discounts, shipping, and taxes in a well-defined order, for a single store.

Operating context. Given a cart (line items with product and quantity), the calculator produces an itemized breakdown: line subtotals, line-level and order-level discounts, shipping, taxes, and a grand total. The calculation is an ordered pipeline of composable stages, and order matters (tax is computed after discounts). Volatile inputs — tax rates, shipping quotes, and already-resolved discounts — come from ports rather than being computed inline. The result must be deterministic and explainable, rounding must happen in one place, and pricing is read-heavy and concurrent. The whole thing is a single in-process component.

Out of scope. Authoring promotion/coupon rules (a stage consumes discounts that are already resolved), the payment step, inventory reservation, currency FX conversion, and persistence.

What to produce. The class hierarchy (Cart, LineItem, the pricing-stage abstraction and pipeline, the price-breakdown result, a Money value object, and the calculator facade), the public API each exposes, and the ordered stage flow. Be explicit about: how stages compose and can be reordered, how the breakdown stays explainable, and how rounding is centralized so no stage introduces float drift.