curl --request POST \
  --url https://api.atlas.kitchen/storefronts/v1/cart/items \
  --header 'Content-Type: application/json' \
  --header 'X-Channel-Id: <api-key>' \
  --data '
{
  "item_id": 123,
  "quantity": 2,
  "notes": "<string>",
  "sub_items": [
    {
      "item_id": 123,
      "modifier_id": 123,
      "item_modifier_group_id": 123,
      "quantity": 2
    }
  ]
}
'

{
  "id": 123,
  "item_id": 123,
  "name": "<string>",
  "quantity": 123,
  "price_cents": 123,
  "calculated_subtotal": 123,
  "per_unit_quantity": 123,
  "unit_label": "<string>",
  "notes": "<string>",
  "modifier_id": 123,
  "item_modifier_group_id": 123,
  "sub_items": [
    {
      "item_id": 123,
      "name": "<string>",
      "quantity": 123,
      "price_cents": 123,
      "calculated_subtotal": 123,
      "per_unit_quantity": 123,
      "unit_label": "<string>",
      "notes": "<string>",
      "modifier_id": 123,
      "item_modifier_group_id": 123,
      "sub_items": "<array>"
    }
  ]
}