---
title: "Sandbox: Mail a Checkbook"
method: POST
path: "/simulations/checkbooks/{checkbook_id}/mail"
---

# Sandbox: Mail a Checkbook

`POST /simulations/checkbooks/{checkbook_id}/mail`

## Request

- Base URL: `https://api.increase.com`
- URL: `https://api.increase.com/simulations/checkbooks/{checkbook_id}/mail`
- Auth: HTTP bearer
- TypeScript: <https://skmtc.dev/increase/apis/increase-api-2/docs/simulations/checkbooks/:checkbook_id/mail/post.ts> (a function with typed inputs; its response is checked against the schema)

Simulates the mailing of a [Checkbook](#checkbooks), which happens periodically throughout the day in production but can be sped up in sandbox. This Checkbook must have a `status` of `pending_submission` or `pending_mailing`.

## Path parameters

- `checkbook_id` string, required: The identifier of the Checkbook you wish to mail.

## Response `200`

Checkbook

- Checkbook: Checkbooks are sets of blank checks that Increase prints and mails on your behalf.
  - `account_number_id` string, required: The identifier of the Account Number the Checkbook is attached to.
  - `created_at` string, date-time, required: The [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) date and time at which the Checkbook was created.
  - `id` string, required: The Checkbook's identifier.
  - `idempotency_key` string, nullable, required: The idempotency key you chose for this object. This value is unique across Increase and is used to ensure that a request is only processed once. Learn more about [idempotency](https://increase.com/documentation/idempotency-keys).
  - `mailing_address` object, required: The destination address that the Checkbook will be mailed to.
    - `city` string, required: The city of the destination address.
    - `line1` string, required: The first line of the destination address.
    - `line2` string, nullable, required: The second line of the destination address.
    - `name` string, required: The recipient at the destination address.
    - `postal_code` string, required: The postal code of the destination address.
    - `state` string, required: The state of the destination address.
  - `payer` object[], required: The payer information printed on the top-left corner of each check.
    - `contents` string, required: The contents of the line.
  - `return_address` object, required: The address the Checkbook will return to if there is a delivery issue.
    - `city` string, required: The city of the return address.
    - `line1` string, required: The first line of the return address.
    - `line2` string, nullable, required: The second line of the return address.
    - `name` string, required: The recipient at the return address.
    - `postal_code` string, required: The postal code of the return address.
    - `state` string, required: The state of the return address.
  - `status` 'pending_submission' | 'pending_mailing' | 'mailed' | 'stopped', required: The lifecycle status of the Checkbook.
  - `style` 'three_ring_binder' | 'folder', required: The physical format of the Checkbook.
  - `tracking_updates` object[], required: Tracking updates relating to the Checkbook's delivery.
    - `category` 'in_transit' | 'processed_for_delivery' | 'delivered' | 'delivery_issue' | 'returning_to_sender' | 'returned_to_sender', required: The type of tracking event.
    - `country` string, required: The ISO 3166-1 alpha-2 country code for the country where the event took place.
    - `created_at` string, date-time, required: The [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) date and time at which the tracking event took place.
    - `postal_code` string, required: The postal code where the event took place.
  - `type` 'checkbook', required: A constant representing the object's type. For this resource it will always be `checkbook`.

## Other responses

- `4XX`: Error
- `5XX`: Error

## Changes

- **2026-11-07** `1fd30bec4150`: 1 info
  - endpoint added

[Operation history](https://skmtc.dev/increase/apis/increase-api-2/changes/simulations/checkbooks/:checkbook_id/mail/post.md)

---

[API](https://skmtc.dev/increase/apis/increase-api-2.md) - [All operations](https://skmtc.dev/increase/apis/increase-api-2/llms.txt) - [OpenAPI document](https://skmtc.dev/increase/apis/increase-api-2/revisions/1fd30bec4150?raw)