Tax rate is applied correctly based on customer location jurisdiction

A single hardcoded tax rate applied to every location — 8.5% regardless of whether the customer is in California, a no-sales-tax state like Oregon, or a VAT jurisdiction in the EU — is both a CWE-682 calculation error and a regulatory compliance failure. South Dakota v. Wayfair (2018) established economic nexus, meaning multi-state sellers must collect destination-state rates. EU VAT Directive 2006/112/EC imposes separate VAT obligations for digital and physical goods sold in EU member states. Overcharging customers in no-tax jurisdictions exposes the business to refund liability; undercharging in mandatory-tax states exposes it to audit penalties and back-tax assessments.

Implement jurisdiction-aware tax lookup in lib/tax.ts using a tax service API or a local rate table with at least 5 distinct jurisdiction entries:

// lib/tax.ts
async function getTaxRate(address: Address): Promise<number> {
  // Option 1: TaxJar (handles Wayfair nexus + VAT automatically)
  const result = await taxjar.rates.get(address.zip, {
    state: address.state,
    country: address.country,
  })
  return result.combined_rate

  // Option 2: Local table (must cover all states you serve)
  const key = `${address.country}-${address.state}`
  return TAX_RATES[key] ?? 0
}

Never return a hardcoded constant from getTaxRate. If using a local table, ensure it includes at least your nexus states and correctly maps zero-rate jurisdictions (OR, MT, NH, DE, AK) to 0.

• ID: ecommerce-shipping-tax.tax-computation.jurisdiction-detection

• Severity: critical

• What to look for: Before evaluating, extract and quote the tax rate lookup function. Count the number of jurisdiction parameters used: (1) state/province, (2) city, (3) zip code, (4) country. Classify whether rates come from a tax service API, a local lookup table, or a hardcoded constant.

• Pass criteria: Tax calculation uses at least 1 jurisdiction parameter (state, province, zip, or country) from the customer's shipping address to determine the applicable tax rate. Rate lookup is performed via a tax service API integration (TaxJar, Avalara, Stripe Tax) or a local rate table with at least 5 distinct rates for different jurisdictions.

• Fail criteria: No jurisdiction detection found, or tax rate is a single hardcoded percentage applied to all locations regardless of address. Must not pass when a function accepts an address parameter but ignores it and returns a constant rate.

• Skip (N/A) when: The business does not collect sales tax (search for tax-related functions, TaxJar/Avalara dependencies, and tax rate constants; if none found, skip).

• Detail on fail: "Tax rate hardcoded at 8.5% in lib/tax.ts line 12. Function accepts address parameter but ignores it — returns 0.085 for all locations." or "Tax lookup table has only 1 entry (US: 8%). No state-level differentiation."

• Cross-reference: Related to ecommerce-shipping-tax.regional-compliance.regional-tax-rules (multi-state rules build on jurisdiction detection) and ecommerce-shipping-tax.checkout-integration.tax-consistency (displayed tax must use same rate logic).

• Remediation: Implement jurisdiction-aware tax calculation in lib/tax.ts:

  // lib/tax.ts
  async function getTaxRate(address: Address): Promise<number> {
    // Option 1: TaxJar integration
    const taxJarRate = await taxjar.rates.get(address.zip, {
      state: address.state,
      country: address.country
    })
    return taxJarRate.combined_rate
  
    // Option 2: Local lookup table
    const key = `${address.country}-${address.state}`
    return TAX_RATES[key] || 0
  }
  
  function calculateTax(subtotal: number, address: Address, items: OrderItem[]): number {
    const taxableSubtotal = items.reduce((sum, item) => {
      return sum + (item.taxable ? item.price * item.quantity : 0)
    }, 0)
  
    const rate = getTaxRate(address)
    return Math.round(taxableSubtotal * rate)
  }