Cart displays accurate item count and subtotal

A cart that displays the wrong item count or a subtotal that double-counts tax erodes purchase confidence at the exact moment it matters most. ISO 25010:2011 functional correctness requires that computed outputs match specifications — a subtotal inflated by tax inclusion makes the cart total contradict the checkout total, producing customer support tickets, chargebacks, and cart abandonment. CWE-20 (Improper Input Validation) also applies when reduction logic operates on unvalidated price data that can produce negative or overflow subtotals.

Fix the total calculation in src/components/CartSummary.tsx to derive count from quantity sums and subtotal from line item products only — never include tax in the reduce:

// src/components/CartSummary.tsx
const itemCount = items.reduce((sum, item) => sum + item.quantity, 0)
const subtotal = items.reduce((sum, item) => sum + (item.price * item.quantity), 0)

If totals are displayed in more than one location (header badge, drawer, cart page), drive all of them from a single shared selector or context value so a quantity update in one location propagates everywhere without a page reload.

• ID: ecommerce-cart-ux.cart-management.cart-totals

• Severity: low

• What to look for: Count all locations where cart totals are displayed (cart page, cart sidebar/drawer, header badge, checkout summary). For each location, verify: (a) item count equals the sum of all line item quantities, (b) subtotal equals the sum of (unit price x quantity) for each item. Quote the calculation code — e.g., items.reduce((sum, item) => sum + item.quantity, 0) in src/components/CartSummary.tsx. Check that totals update reactively when items change without requiring a page reload.

• Pass criteria: Cart displays correct count (sum of quantities) and correct subtotal (sum of line item totals) in at least 1 location. Totals update in real-time as items are added, removed, or updated. No more than 0 display locations should show stale or incorrect totals. Report the count: "X total-display locations found, all X show accurate values."

• Fail criteria: Displayed count or subtotal does not match actual items (e.g., shows 3 items but cart has 5, or subtotal double-counts tax).

• Skip (N/A) when: Cart UI does not display a count or subtotal anywhere (no header badge, no cart summary component).

• Detail on fail: Example: "Cart shows '3 items' but contains 5 items. Subtotal calculation in src/components/CartSummary.tsx line 8 includes tax in the reduce, double-counting. 1 of 2 display locations shows incorrect totals."

• Cross-reference: For the pre-payment order summary accuracy, see the pre-payment-review check in the Checkout Flow category.

• Remediation: Fix total calculations in your cart summary, typically at src/components/CartSummary.tsx:

  // src/components/CartSummary.tsx
  function CartSummary({ items }) {
    const itemCount = items.reduce((sum, item) => sum + item.quantity, 0)
    const subtotal = items.reduce((sum, item) => sum + (item.price * item.quantity), 0)
  
    return (
      <div>
        <p>Items: {itemCount}</p>
        <p>Subtotal: ${(subtotal / 100).toFixed(2)}</p>
      </div>
    )
  }