Shipping costs are taxed appropriately per jurisdiction

Charging sales tax on shipping in Texas (which does not tax separately-stated shipping charges) overcharges every Texas customer on every order. Exempting shipping tax in New York (which does tax it) under-collects on every New York order. US state shipping taxability rules differ by jurisdiction and cannot be flattened to a single global policy. EU VAT Directive 2006/112/EC treats shipping as part of the taxable supply for EU transactions, requiring a third distinct rule. A store with no jurisdiction-specific shipping tax configuration and no documentation of its shipping tax policy is applying an undocumented guess to every order.

Document and enforce jurisdiction-aware shipping tax in lib/tax.ts with a named configuration map:

// lib/tax.ts
// US state shipping taxability — source: multistate tax commission, 2024.
// When in doubt, use a tax service API (TaxJar/Avalara) to avoid maintaining this manually.
const SHIPPING_TAX_RULES: Record<string, boolean> = {
  'US-NY': true,  // NY taxes shipping when delivered by vendor
  'US-IL': true,
  'US-PA': true,
  'US-TX': false, // TX exempts separately-stated shipping
  'US-CA': false,
  'EU':    true,  // VAT applies to shipping in EU
}

function isShippingTaxable(address: Address): boolean {
  const key = address.country === 'US'
    ? `US-${address.state}`
    : address.country === 'EU' ? 'EU' : null
  return key ? (SHIPPING_TAX_RULES[key] ?? false) : false
}

Reference isShippingTaxable inside buildTaxableSubtotal rather than hardcoding a global include/exclude assumption.