cartId

window._upstack('track', 'add_to_cart', {
  // Standard parameters
  value: 69.98,
  currency: 'USD',
  items: [{
    id: '8012345678',
    variant: '44120033',
    variantName: 'Blue / Medium',
    name: 'Classic Cotton Tee',
    sku: 'TEE-BLU-M',
    price: 34.99,
    quantity: 2,
    brand: 'Example Brand',
    category: 'Apparel',
    url: 'https://example.com/products/classic-tee',
    imageUrl: 'https://example.com/images/classic-tee.jpg'
  }],
  cartId: 'cart_5521',

  // User data
  email: 'jane@example.com',

  // Custom data
  addedFrom: 'quick_view',
  bundleId: 'bundle_77'
});