curl --request POST \
  --url https://api.headlesscommerce.io/v1/storefront/carts/{id}/merge \
  --header 'Authorization: Bearer <token>' \
  --header 'X-Customer-Token: <api-key>'

{
  "id": "<string>",
  "status": "active",
  "session_id": "<string>",
  "customer_id": "<string>",
  "items": [
    {
      "id": "<string>",
      "variant_id": "<string>",
      "product_id": "<string>",
      "product_name": "<string>",
      "variant_name": "<string>",
      "sku": "<string>",
      "image_url": "<string>",
      "quantity": 123,
      "unit_price": {
        "amount": 123,
        "currency": "<string>"
      },
      "line_discount": {
        "amount": 123,
        "currency": "<string>"
      },
      "line_total": {
        "amount": 123,
        "currency": "<string>"
      },
      "metadata": {}
    }
  ],
  "shipping_address": {
    "name": "<string>",
    "company": "<string>",
    "line1": "<string>",
    "line2": "<string>",
    "city": "<string>",
    "state": "<string>",
    "postal_code": "<string>",
    "country": "<string>",
    "phone": "<string>"
  },
  "billing_address": {
    "name": "<string>",
    "company": "<string>",
    "line1": "<string>",
    "line2": "<string>",
    "city": "<string>",
    "state": "<string>",
    "postal_code": "<string>",
    "country": "<string>",
    "phone": "<string>"
  },
  "shipping_method_id": "<string>",
  "discount_codes": [
    "<string>"
  ],
  "summary": {
    "subtotal": {
      "amount": 123,
      "currency": "<string>"
    },
    "discount_total": {
      "amount": 123,
      "currency": "<string>"
    },
    "shipping_total": {
      "amount": 123,
      "currency": "<string>"
    },
    "tax_total": {
      "amount": 123,
      "currency": "<string>"
    },
    "total": {
      "amount": 123,
      "currency": "<string>"
    },
    "item_count": 123,
    "total_quantity": 123
  },
  "applied_discounts": [
    {
      "discount_id": "<string>",
      "code": "<string>",
      "name": "<string>",
      "value_type": "fixed",
      "value": 123,
      "saved": {
        "amount": 123,
        "currency": "<string>"
      }
    }
  ],
  "metadata": {},
  "created_at": "2023-11-07T05:31:56Z",
  "updated_at": "2023-11-07T05:31:56Z"
}