# Dynamically adjust the payment amount

You can dynamically adjust the payment amount based on changes during checkout. For example, if the customer applies a discount code or changes their shipping method.

---

## Modify the payment sheet amount `Client side`

> **Information**
>
> You can only modify the payment sheet amount for Apple Pay and Google Pay payments.

During checkout, Apple Pay and Google Pay display their payment sheet with the payment amount to the customer.

To modify the payment amount shown, call the `update()` on your `CheckoutComponent` instance instance and specify a new amount:

**Android**

```java
paymentMethodComponent.update(UpdateDetails(amount = amount))
```

**iOS**

```swift
try checkoutComponentsSDK.update(with: .init(amount: 200))
```

If you update the payment sheet amount, you must also add a `handleSubmit` callback to the initialization code for your Flow instance to handle the payment attempt.

---

## Add the handleSubmit event `Client side`

The `handleSubmit` [event](https://www.checkout.com/docs/payments/accept-payments/accept-a-payment-on-your-mobile-app/flow-for-mobile-library-reference/ios/componentcallback.md#handleSubmit(SessionData)_:_APICallResult) enables you to submit a modified payment session.

Add the event to the initialization code for your Flow for mobile configuration.

Implement the callback logic to provide the payment submission data to your server-side integration:

**Android**

```java
suspend fun handleSubmit(sessionData: SessionData): ApiCallResult {
    handlePaymentSubmission()
}
val callback = ComponentCallback(handleSubmit = ::handleSubmit)
```

**iOS**

```swift
let callbacks = CheckoutComponents.Callbacks(handleSubmit: { sessionData in
    submitPaymentSession(sessionData)
  })

let configuration = CheckoutComponents.Configuration(..., callbacks: callbacks)
```

---

## Submit the modified payment session `Server side`

Call the [Submit a payment session](https://api-reference.checkout.com/tag/Flow/#operation/SubmitPaymentSession) endpoint and provide the payment session ID as the `{id}` path parameter.

In your request, provide the payment session data from the `handleSubmit` event in `session_data`. You can also modify any of the following fields:

- `3ds`
- `amount`
- `items`
- `reference`

> **Information**
>
> Your base URL's `{prefix}` value is unique to your account and environment. To learn how to retrieve your base URLs for the sandbox and production environments, see [API endpoints](https://www.checkout.com/docs/developer-resources/api/api-endpoints.md).

**Production**

`POST` `https://{prefix}.api.checkout.com/payment-sessions/{id}/submit`

**Sandbox**

`POST` `https://{prefix}.api.sandbox.checkout.com/payment-sessions/{id}/submit`

### Request example

```json
{
  "session_data": "{SESSION_DATA_FROM_FLOW}",
  "3ds": {
    "enabled": true
  }
}
```

### Response example

```json
{
  "id": "pay_f2ws3hn4ijquzbytp3v5lsfxge",
  "status": "Action Required",
  "type": "card",
  "action": {
    "type": "3ds",
    "url": "http://3ds2.checkout.com/interceptor/sid_y3oqhf46pyzuxjbcn2giaqnb44"
  }
}
```

---

## Handle the payment response `Client side`

You must provide the unmodified response to the `handleSubmit` event in the client-side integration. Flow handles any additional required actions.