# Server-side callbacks

This page describes server-side callbacks available with Kustom Checkout (KCO).

## How it's done

These callbacks allow merchants to update order details during the checkout process, for example, when a customer changes their address or the order needs validation.

### Address update

If you include the `address_update` URL in the `merchant_urls` object when creating the order, a POST request will be sent to this URL when the customer updates their shipping address.


```json
  "merchant_urls": {
    "terms": "https://merchant.com/toc",
    "checkout": "https://merchant.com/checkout?kustom_order_id={checkout.order.id}",
    "confirmation": "https://merchant.com/confirmation?kustom_order_id={checkout.order.id}",
    "push": "https://merchant.com/push?kustom_order_id={checkout.order.id}",
    "address_update": "https://merchant.com/address_update?kustom_order_id={checkout.order.id}"
  }
```

This callback can be used to update tax and shipping costs, and shipping options.

[Link to API specification](/contents/api/checkout-callback)

### Country change


```json
  "merchant_urls": {
    "terms": "https://merchant.com/toc",
    "checkout": "https://merchant.com/checkout?kustom_order_id={checkout.order.id}",
    "confirmation": "https://merchant.com/confirmation?kustom_order_id={checkout.order.id}",
    "push": "https://merchant.com/push?kustom_order_id={checkout.order.id}",
    "country_change": "https://merchant.com/country_change?kustom_order_id={checkout.order.id}"
  }
```

[Link to API specification](/contents/api/checkout-callback)

### Shipping option update

The `shipping_option_update` callback is used to adjust the order total based on the selected shipping option, updating both the `total_amount` and `order_lines`.


```json
  "merchant_urls": {
    "terms": "https://merchant.com/toc",
    "checkout": "https://merchant.com/checkout?kustom_order_id={checkout.order.id}",
    "confirmation": "https://merchant.com/confirmation?kustom_order_id={checkout.order.id}",
    "push": "https://merchant.com/push?kustom_order_id={checkout.order.id}",
    "shipping_option_update": "https://merchant.com/shipping_option_update?kustom_order_id={checkout.order.id}"
  }
```

This is only relevant if the shipping selector is within KCO. Otherwise, KCO automatically handles shipping cost updates.

[Link to API specification](/contents/api/checkout-callback)

### Order validation

The validation callback allows you to perform final order validation before purchase completion. See the [validate order](/contents/checkout/use-cases/validate-order) page for more details.

[Link to API specification](/contents/api/checkout-callback)