Add Shipping Method

curl -X POST 'http://localhost:9000/store/carts/cart_01JB2KB1CD/shipping-methods' \
  -H 'Content-Type: application/json' \
  -H 'x-publishable-api-key: pk_01JB2K3XYZ' \
  -d '{"option_id": "so_01JB2KC4GH"}'

const { cart } = await client.store.carts.$id.shippingMethods.mutate({
  $id: "cart_01JB2KB1CD",
  option_id: "so_01JB2KC4GH",
})

{
  "cart": {
    "id": "cart_01JB2KB1CD",
    "currency_code": "eur",
    "items": [{ "id": "cali_01JB2KB3EF", "quantity": 1, "unit_price": 4500 }],
    "shipping_methods": [
      {
        "shipping_option_id": "so_01JB2KC4GH",
        "amount": 900,
        "is_tax_inclusive": false,
        "tax_lines": []
      }
    ],
    "shipping_total": 900,
    "total": 5400
  }
}