---
title: "Add an item/add-on"
description: "Prorated charge."
canonical_url: "https://twominutebilling.com/help/developers/api-reference/subscriptions/add-an-item-add-on"
---
# Add an item/add-on

> Prorated charge.

<api-operation method="POST" path="/v1/businesses/{biz}/subscriptions/{sub}/items">
<template v-slot:docs="">

## Path parameters

<table>
<thead>
  <tr>
    <th>
      Parameter
    </th>
    
    <th>
      Type
    </th>
    
    <th>
      Required
    </th>
    
    <th>
      Description
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <code>
        biz
      </code>
    </td>
    
    <td>
      <code>
        string
      </code>
    </td>
    
    <td>
      <strong>
        yes
      </strong>
    </td>
    
    <td>
      Business id (biz_…)
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        sub
      </code>
    </td>
    
    <td>
      <code>
        string
      </code>
    </td>
    
    <td>
      <strong>
        yes
      </strong>
    </td>
    
    <td>
      Subscription id (sub_…)
    </td>
  </tr>
</tbody>
</table>

## Headers

<table>
<thead>
  <tr>
    <th>
      Parameter
    </th>
    
    <th>
      Type
    </th>
    
    <th>
      Required
    </th>
    
    <th>
      Description
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <code>
        Idempotency-Key
      </code>
    </td>
    
    <td>
      <code>
        string
      </code>
    </td>
    
    <td>
      no
    </td>
    
    <td>
      Safely retry writes — repeats return the original response. If a write FAILED after it had already taken effect, that failure is what repeats: the key is spent, and a new attempt needs a new key.
    </td>
  </tr>
</tbody>
</table>

## Body

The request body is **required**.

<table>
<thead>
  <tr>
    <th>
      Field
    </th>
    
    <th>
      Type
    </th>
    
    <th>
      Required
    </th>
    
    <th>
      Description
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <code>
        price
      </code>
    </td>
    
    <td>
      <code>
        string
      </code>
    </td>
    
    <td>
      <strong>
        yes
      </strong>
    </td>
    
    <td>
      The price's id (a slug id, e.g. <code>
        pro-monthly-usd
      </code>
      
      ).
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        quantity
      </code>
    </td>
    
    <td>
      <code>
        integer
      </code>
    </td>
    
    <td>
      no
    </td>
    
    <td>
      The quantity. Defaults to <code>
        1
      </code>
      
      .
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        unit_price_minor
      </code>
    </td>
    
    <td>
      <code>
        integer
      </code>
    </td>
    
    <td>
      no
    </td>
    
    <td>
      Custom price override (flat/per-unit only), minor units
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        invoice_immediately
      </code>
    </td>
    
    <td>
      <code>
        boolean
      </code>
    </td>
    
    <td>
      no
    </td>
    
    <td>
      Where the prorated charge lands — the same Invoicing Option POST …/change carries. true raises the invoice now and collects it; false queues the line as an unbilled charge folded into the next renewal invoice (where tax and account credits are applied), and the response then carries <code>
        unbilled_charges
      </code>
      
       instead of <code>
        proration_invoice
      </code>
      
      . OMITTED resolves the business setting <code>
        default_invoice_immediately
      </code>
      
       (off by default), exactly as /change does — so a composition edit applied as several calls lands on one answer.
    </td>
  </tr>
</tbody>
</table>

## Response

<table>
<thead>
  <tr>
    <th>
      Field
    </th>
    
    <th>
      Type
    </th>
    
    <th>
      Description
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <code>
        object
      </code>
    </td>
    
    <td>
      <code>
        string
      </code>
    </td>
    
    <td>
      The object type; always <code>
        subscription_item
      </code>
      
      . One of <code>
        subscription_item
      </code>
      
      .
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        id
      </code>
    </td>
    
    <td>
      <code>
        string
      </code>
    </td>
    
    <td>
      The subscription item id.
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        price
      </code>
    </td>
    
    <td>
      <code>
        string
      </code>
    </td>
    
    <td>
      The price id.
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        quantity
      </code>
    </td>
    
    <td>
      <code>
        integer
      </code>
    </td>
    
    <td>
      The quantity.
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        proration_invoice
      </code>
    </td>
    
    <td>
      <code>
        object
      </code>
    </td>
    
    <td>
      The invoice that the change raised, or null if it raised none.
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        unbilled_charges
      </code>
    </td>
    
    <td>
      <code>
        array of object
      </code>
    </td>
    
    <td>
      Present only when the charge was queued instead of invoiced.
    </td>
  </tr>
</tbody>
</table>

## Status codes

<table>
<thead>
  <tr>
    <th>
      Status
    </th>
    
    <th>
      Meaning
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <code>
        201
      </code>
    </td>
    
    <td>
      Item added
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        400
      </code>
    </td>
    
    <td>
      Invalid
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        404
      </code>
    </td>
    
    <td>
      No such subscription/price
    </td>
  </tr>
  
  <tr>
    <td>
      <code>
        409
      </code>
    </td>
    
    <td>
      Idempotency-Key reused with a different payload, or a request with it is still in progress
    </td>
  </tr>
</tbody>
</table>
</template>

<template v-slot:request="">

```bash [Request]
curl -X POST "$TMBILLING_URL/v1/businesses/biz_XXXXXXXXXXXX01/subscriptions/sub_XXXXXXXXXXXX01/items" \
  -H "Authorization: Bearer $TMBILLING_API_KEY" \
  -H "Idempotency-Key: $(uuidgen)" \
  -H "Content-Type: application/json" \
  -d '{
    "price": "pro-monthly-usd",
    "quantity": 1
  }'
```

</template>

<template v-slot:response="">
<code-group>

```json [201 Created]
{
  "object": "subscription_item",
  "id": "si_XXXXXXXXXXXX01",
  "price": "extra-seat-monthly-usd",
  "quantity": 2,
  "proration_invoice": {
    "id": "inv_XXXXXXXXXXXX01",
    "status": "paid",
    "total_minor": 4900
  },
  "unbilled_charges": [
    {
      "id": "uch_XXXXXXXXXXXX01",
      "amount_minor": 2500
    }
  ]
}
```

```json [400 Bad Request]
{
  "error": {
    "type": "invalid_request_error",
    "message": "Invalid"
  }
}
```

```json [404 Not Found]
{
  "error": {
    "type": "invalid_request_error",
    "message": "No such subscription/price"
  }
}
```

```json [409 Conflict]
{
  "error": {
    "type": "idempotency_error",
    "message": "Idempotency-Key reused with a different payload, or a request with it is still in progress"
  }
}
```

</code-group>
</template>
</api-operation>


## Sitemap

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