> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mercurjs.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Add Line Item

> Add a seller offer to the cart as a line item.

Adds an item to the cart by offer — Mercur resolves the offer's variant and stamps the `offer_id` on the line item so the cart can later split into per-seller orders.

<Note>
  Unlike vanilla Medusa, this route takes an `offer_id` instead of a `variant_id`; an unknown offer returns a `404`.
</Note>

## Path parameters

<ParamField path="id" type="string" required>
  The cart's ID.
</ParamField>

## Body parameters

<ParamField body="offer_id" type="string" required>
  ID of the offer to add.
</ParamField>

<ParamField body="quantity" type="number" required>
  Quantity to add; must be a positive integer.
</ParamField>

<ParamField body="unit_price" type="number">
  Custom unit price overriding the offer's calculated price.
</ParamField>

<ParamField body="compare_at_unit_price" type="number">
  Compare-at unit price shown as the original price.
</ParamField>

<ParamField body="metadata" type="object">
  Custom key-value data stored on the line item; `offer_id` is merged in automatically.
</ParamField>

<ParamField body="additional_data" type="object">
  Data passed to the add-to-cart workflow's hooks.
</ParamField>

## Response

<ResponseField name="cart" type="object">
  <Expandable title="properties">
    <ResponseField name="id" type="string">The cart's ID.</ResponseField>
    <ResponseField name="currency_code" type="string">The cart's currency.</ResponseField>
    <ResponseField name="region_id" type="string">The cart's region.</ResponseField>
    <ResponseField name="email" type="string | null">The customer's email.</ResponseField>
    <ResponseField name="items" type="object[]">Line items, each carrying `offer_id` in its `metadata`.</ResponseField>
    <ResponseField name="shipping_methods" type="object[]">Selected shipping methods.</ResponseField>
    <ResponseField name="promotions" type="object[]">Applied promotions.</ResponseField>
    <ResponseField name="shipping_address" type="object | null">The shipping address.</ResponseField>
    <ResponseField name="billing_address" type="object | null">The billing address.</ResponseField>
    <ResponseField name="payment_collection" type="object | null">The cart's payment collection with its sessions.</ResponseField>
    <ResponseField name="total" type="number">Cart total including tax and shipping.</ResponseField>
    <ResponseField name="subtotal" type="number">Cart subtotal.</ResponseField>
    <ResponseField name="tax_total" type="number">Total tax amount.</ResponseField>
    <ResponseField name="discount_total" type="number">Total discount amount.</ResponseField>
    <ResponseField name="shipping_total" type="number">Total shipping amount.</ResponseField>
  </Expandable>
</ResponseField>

<RequestExample>
  ```bash cURL theme={null}
  curl -X POST 'http://localhost:9000/store/carts/cart_01JB2KB1CD/line-items' \
    -H 'Content-Type: application/json' \
    -H 'x-publishable-api-key: pk_01JB2K3XYZ' \
    -d '{"offer_id": "offer_01JB2K6G5H", "quantity": 1}'
  ```

  ```ts JS Client theme={null}
  const { cart } = await client.store.carts.$id.lineItems.mutate({
    $id: "cart_01JB2KB1CD",
    offer_id: "offer_01JB2K6G5H",
    quantity: 1,
  })
  ```
</RequestExample>

<ResponseExample>
  ```json 200 theme={null}
  {
    "cart": {
      "id": "cart_01JB2KB1CD",
      "currency_code": "eur",
      "region_id": "reg_01JB2K4S1T",
      "email": null,
      "items": [
        {
          "id": "cali_01JB2KB3EF",
          "title": "M",
          "product_title": "Linen Shirt",
          "quantity": 1,
          "unit_price": 4500,
          "metadata": { "offer_id": "offer_01JB2K6G5H" }
        }
      ],
      "shipping_methods": [],
      "promotions": [],
      "shipping_address": null,
      "billing_address": null,
      "payment_collection": null,
      "total": 4500,
      "subtotal": 4500,
      "tax_total": 0,
      "discount_total": 0,
      "shipping_total": 0
    }
  }
  ```
</ResponseExample>