## Start Checkout

## Endpoint signature

```http
POST https://api.reservepay.com/merchants/start-checkout HTTP/1.1
Content-Type: application/json
Accept: application/json
Authorization: Bearer <token>

{
  installation_id: string,
  amount: string,
  currency: string,
  return_url: string,
  customer_id: string?,
  customer_email: string?,
  customer_mobile: string?,
  customer_name: string?,
}
```

Returns: object

## Request arguments

### `installation_id` (string)

**Required**. The installation ID for this platform (e.g. your iOS app, Android app, website, or terminal).

### `amount` (string)

**Required**. The payment amount in the smallest currency unit (e.g. cents). Must be a positive integer.

### `currency` (string)

**Required**. The three-letter ISO currency code (e.g. THB). Must match the funding currency configured for your provider.

### `return_url` (string)

**Required**. The URL where we'll redirect your customer after they complete the
payment flow. Should be a page that can handle both successful and
failed payments.

### `customer_id` (string)

_Optional_. Either a Customer ID (starting with `cst_`) or your own external
customer identifier. We'll link this payment to an existing customer
if found, or create a new customer if not. This helps track payment
history per customer.

If you provide your own external ID and a customer with that ID
doesn't exist, we'll create a new customer using that ID as their
external identifier.

### `customer_email` (string)

_Optional_. Customer's email address. Required if the installation is configured to require email capture.

### `customer_mobile` (string)

_Optional_. Customer's mobile phone number. Required if the installation is configured to require mobile capture.

### `customer_name` (string)

_Optional_. Customer's name.

## Response attributes

### `checkout_id` (string)

**Always present**. The unique identifier for the checkout

### `amount` (string)

**Always present**. The payment amount in the smallest currency unit (e.g. cents).

### `currency` (string)

**Always present**. The currency code for the payment amount

### `installation_id` (string)

**Always present**. The unique identifier of the merchant installation associated with this checkout

### `customer_id` (string)

_Nullable_. The unique identifier of the customer making the payment

### `customer_email` (string)

_Nullable_. The email address of the customer

### `customer_mobile` (string)

_Nullable_. The mobile phone number of the customer

### `customer_name` (string)

_Nullable_. The name of the customer

### `redirect_url` (string)

**Always present**. The URL where the customer will be redirected to make payment

### `payment_id` (string)

_Nullable_. The unique identifier of the payment associated with this checkout. When nil, the payment has not been paid yet.

## Errors common to all endpoints

### `UNHANDLED_ERROR`

This error occurs when the server encounters an unexpected internal error that
it cannot handle gracefully. This typically happens due to bugs, infrastructure
issues, or edge cases that weren't anticipated during development.

### `INVALID_ARGUMENTS`

This error occurs when the request contains invalid or missing parameters.
Common cases include missing required fields, or values that don't match the
expected format or type.

### `BAD_VERSION`

This error occurs when making requests to an API version that does not exist.
This commonly happens when using an outdated SDK or when the API version
specified in the request URL is incorrect.