curl --request POST \
  --url https://staging.api.commercengine.io/api/v1/{store_id}/storefront/carts/{id}/items \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "product_id": "01H9XYZ12345ABCDE",
  "variant_id": null,
  "quantity": 1
}'

{
  "message": "<string>",
  "success": true,
  "content": {
    "cart": {
      "id": "<string>",
      "active": true,
      "customer_email": "jsmith@example.com",
      "customer_phone": "+919988776655",
      "customer_note": "<string>",
      "is_promotion_applied": true,
      "is_coupon_applied": true,
      "coupon_code": "<string>",
      "coupon_discount_amount": 1,
      "coupon_discount_percent": 1,
      "promotion_discount_amount": 1,
      "promotion_discount_percent": 1,
      "subtotal": 1,
      "items_tax_amount": 1,
      "subtotal_including_tax": 1,
      "shipping_estimated_cost": 123,
      "shipping_discount_amount": 123,
      "shipping_amount": 123,
      "shipping_tax_rate": 123,
      "shipping_tax_amount": 123,
      "shipping_amount_including_tax": 123,
      "shipping_provider_id": "<string>",
      "shipping_provider_name": "<string>",
      "courier_company_id": "<string>",
      "courier_company_name": "<string>",
      "estimated_delivery_days": 123,
      "handling_charge_excluding_tax": 1,
      "handling_charge_tax_amount": 1,
      "handling_charge_including_tax": 1,
      "total_tax": 1,
      "grand_total": 1,
      "credit_balance_used": 1,
      "to_be_paid": 1,
      "loyalty_points_redeemed": 1,
      "loyalty_points_earned": 1,
      "cart_items_count": 1,
      "cart_items": [
        {
          "product_id": "<string>",
          "variant_id": "<string>",
          "sku": "SKU1254",
          "slug": "<string>",
          "product_name": "<string>",
          "variant_name": "<string>",
          "product_type": "physical",
          "product_image_url": "<string>",
          "stock_available": true,
          "backorder": true,
          "on_offer": true,
          "on_subscription": true,
          "on_promotion": true,
          "quantity": 123,
          "is_free_item": true,
          "free_quantity": 123,
          "promotion_discount_amount": 123,
          "coupon_discount_amount": 123,
          "price_including_tax": false,
          "listing_price": 200,
          "selling_price": 199,
          "selling_price_excluding_tax": 123,
          "min_order_quantity": 1,
          "max_order_quantity": 123,
          "incremental_quantity": 1,
          "handling_charge_excluding_tax": 1,
          "handling_charge_tax_rate": 1,
          "handling_charge_including_tax": 1,
          "tax_type": "GST",
          "tax_rate": 123,
          "tax_amount": 123,
          "associated_options": {
            "background_color": {
              "name": "Background Color",
              "value": {
                "name": "Blue",
                "hexcode": "#0000FF"
              },
              "type": "color"
            },
            "size": {
              "name": "Size",
              "value": "Large",
              "type": "single-select"
            }
          },
          "attributes": [
            {
              "id": "<string>",
              "name": "<string>",
              "key": "<string>",
              "type": "color",
              "value": [
                {
                  "name": "<string>",
                  "hexcode": "<string>"
                }
              ]
            }
          ],
          "subscriptions": [
            {
              "id": "<string>",
              "subscription_plan": "<string>",
              "subscription_price": 123,
              "billing_interval": 123,
              "billing_frequency": "monthly",
              "billing_limit": 123,
              "fulfill_separately": false,
              "order_interval": 123,
              "order_frequency": "monthly",
              "order_limit": 123,
              "minimum_quantity": 123,
              "billing_trial_days": 123
            }
          ]
        }
      ],
      "billing_address": {
        "id": "<string>",
        "first_name": "<string>",
        "last_name": "<string>",
        "country_code": "<string>",
        "phone": "<string>",
        "email": "<string>",
        "address_line1": "<string>",
        "address_line2": "<string>",
        "landmark": "<string>",
        "pincode": "<string>",
        "city": "<string>",
        "state": "<string>",
        "country": "<string>",
        "tax_identification_number": "123456789",
        "business_name": "<string>",
        "is_phone_verified": true,
        "is_email_verified": true
      },
      "shipping_address": {
        "id": "<string>",
        "first_name": "<string>",
        "last_name": "<string>",
        "country_code": "<string>",
        "phone": "<string>",
        "email": "<string>",
        "address_line1": "<string>",
        "address_line2": "<string>",
        "landmark": "<string>",
        "pincode": "<string>",
        "city": "<string>",
        "state": "<string>",
        "country": "<string>",
        "tax_identification_number": "123456789",
        "business_name": "<string>",
        "is_phone_verified": true,
        "is_email_verified": true
      },
      "currency": {
        "name": "Indian Rupee",
        "code": "INR",
        "symbol": "₹"
      },
      "metadata": {},
      "expires_at": "2023-11-07T05:31:56Z",
      "applied_promotions": [
        {
          "promotion_id": "<string>",
          "promotion_type": "discount",
          "savings": 123,
          "product_id": "<string>",
          "variant_id": "<string>",
          "product_name": "<string>",
          "variant_name": "<string>",
          "applied_sequence": 123,
          "scope": "<string>"
        }
      ],
      "applied_coupons": [
        {
          "coupon_id": "<string>",
          "coupon_type": "discount",
          "savings": 123,
          "product_id": "<string>",
          "variant_id": "<string>",
          "product_name": "<string>",
          "variant_name": "<string>"
        }
      ],
      "promotion_savings": 1,
      "coupon_savings": 1,
      "other_savings": 1,
      "total_savings": 1
    }
  }
}