Tax computation excludes shipping costs unless jurisdiction requires shipping tax

Approximately 12 US states tax shipping charges; the remaining 38 do not. When shipping is always included in the taxable subtotal — or always excluded with no jurisdiction check — the store either overcharges customers in non-taxing states or under-collects tax in states like New York, Illinois, and Pennsylvania (CWE-682, ISO 25010:2011 functional correctness). The EU VAT Directive 2006/112/EC treats shipping as part of the taxable supply, applying a different rule than most US states. US state shipping taxability rules are jurisdiction-specific and cannot be flattened to a global default. When no code comment documents the shipping tax policy, the decision becomes invisible and unauditble.

Implement jurisdiction-aware shipping tax in lib/tax.ts with a documented configuration object:

// lib/tax.ts
// Source: state revenue authority rulings. Last reviewed: 2024-01.
// US states that tax shipping charges as of 2024:
const STATES_TAXING_SHIPPING = new Set(['NY', 'IL', 'PA', 'WA', 'GA', 'IN', 'TX', 'KS'])

function buildTaxableSubtotal(
  items: OrderItem[],
  shippingCents: number,
  address: Address
): number {
  const productSubtotal = items
    .filter(i => !i.is_tax_exempt)
    .reduce((sum, i) => sum + i.price * i.quantity, 0)

  const taxShipping = address.country === 'US'
    ? STATES_TAXING_SHIPPING.has(address.state)
    : true // EU VAT applies to shipping

  return productSubtotal + (taxShipping ? shippingCents : 0)
}

Replace any global taxableSubtotal = productSubtotal + shippingCents with this function and add a comment citing the source of the jurisdiction list.