Shipping rates are calculated accurately based on weight, distance, or method

A flat shipping rate applied to every order regardless of weight, destination, or carrier method is a CWE-682 (Incorrect Calculation) defect that costs money in both directions: the store eats losses on heavy or long-distance shipments, or overcharges customers on lightweight local orders — which triggers chargebacks and cart abandonment. ISO 25010:2011 functional correctness requires that computed outputs match business rules. When every order is charged the same $5.00 regardless of a 50 lb appliance vs. a greeting card, the store is either subsidizing shipping losses or alienating customers who see inflated charges and leave.

Implement dynamic shipping calculation in lib/shipping.ts that uses at least 2 of 4 factors — weight, destination zone, method, or carrier API:

// lib/shipping.ts
async function calculateShippingCost(
  items: OrderItem[],
  destination: Address,
  method: ShippingMethod
): Promise<number> {
  const totalWeight = items.reduce((sum, item) => sum + item.weight * item.quantity, 0)

  if (method === 'carrier') {
    const shipment = await easypost.shipment.create({
      from_address: { city: ORIGIN_CITY, state: ORIGIN_STATE },
      to_address: { city: destination.city, state: destination.state },
      parcel: { weight: totalWeight },
    })
    return Number(shipment.rates[0].rate) * 100 // convert to cents
  }

  const base = getBaseRate(destination.state)
  const weightSurcharge = Math.ceil(totalWeight / 5) * 50 // 50¢ per 5 lbs
  return base + weightSurcharge
}

Delete any hardcoded constant returned from a calculateShipping function and replace with this parameterised implementation.