# Line items

Line items can be of different types (anything that can have a cost or add a discount and contribute to changing the total order amount):

* `skus`
* `bundles`
* `shipments`
* `payment_methods`
* `gift_cards`
* `adjustments`
* any valid [promotion](https://docs.commercelayer.io/core-api-reference/promotions) type (e.g. `fixed_amount_promotions`, `percentage_discount_promotions`, etc.)

Create a line item if you want to add an [SKU](https://docs.commercelayer.io/core-api-reference/skus) or [bundle](https://docs.commercelayer.io/core-api-reference/bundles) to an order. Other types of line items are automatically created (e.g. to add a shipping cost to the order or apply a discount). The sum of all line item amounts makes the order's total, split into subtotal, taxes, and discount. The [order](https://docs.commercelayer.io/core-api-reference/orders) object includes all the data you need. No calculation is required on your application.

## Updating quantity

When creating a new line item, set the `_update_quantity` attribute to `true` if you want to update the line item quantity (if present) instead of creating a new line item for the same SKU.

## Overriding amounts

A line item's `unit_amount_cents` and `options_amount_cents` can be assigned a value that differs from the one defined in the market's price list of the associated order or for the SKU option of the associated SKU. To do that, you must use [integration](https://app.gitbook.com/s/-LgByaSP8eKjad-MIuHE/api-credentials#integration) API credentials, otherwise the API will return an error.

Sometimes, you can decide not to manage prices within Commerce Layer but use an [external service](https://app.gitbook.com/s/-LgByaSP8eKjad-MIuHE/external-resources/external-prices). When adding a line item to an order, set the `_external_price` attribute to `true` if you want the line item price to be provided by your external endpoint, instead of Commerce Layer.

If you want to apply a [custom shipping cost policy](https://docs.commercelayer.io/core-api-reference/markets#market-specific-shipping-costs-policy) you can set to 0 the unit amount of some of the line items of type `shipments`. To do that, leverage the dedicated attribute at the market level.

## Reserving the stock

You can temporarily reserve the stock associated with a line item of type `skus` or `bundles` by setting the `_reserve_stock` attribute to `true`. This way, the necessary [stock reservations](https://docs.commercelayer.io/core-api-reference/stock_reservations) are created and the stock is reserved until their expiration date. If the order isn't placed before the expiration date or the line item is removed, the corresponding reserved stock is released. Once the order is placed the stock is reserved until the order is approved or cancelled. The default stock reservation expiration date (**1 hour**) can be changed at the [inventory model](https://docs.commercelayer.io/core-api-reference/inventory_models#stock-reservation-settings) level. Any time you patch a line item passing the `_reserve_stock` attribute, the expiration date of the associated existing stock reservations is updated.

{% hint style="info" %}
Stock reservations act on the whole line item quantity. If the available stock is split between different stock locations, multiple stock reservations are created.
{% endhint %}

{% hint style="warning" %}
For security reasons, only [integrations](https://app.gitbook.com/s/-LgByaSP8eKjad-MIuHE/api-credentials#integration) are allowed to change the `_reserve_stock` attribute of a line item.
{% endhint %}

## Choosing line items that generate subscriptions

Line items that have the `frequency` attribute populated (with one of the frequency values available for the associated market, as set at the [subscription model](https://docs.commercelayer.io/core-api-reference/subscription_models) level) will be used to generate the recurring orders involved in the order subscription process, according to the subscription model's strategy.