## Select payment method

This endpoint creates a new payment session with the customer's chosen payment method.
It returns a payment session ID that you'll use in subsequent API calls to complete the payment flow.

The flow after calling this endpoint depends on the selected payment method:

- For card payments: You must provide a card token obtained from `tokenize-card`
- For QR code payments: Call `retrieve-qr-data` to get the QR code data to display
- For redirect-based payments: Call `get-redirect-url` to get the URL to redirect the customer to

Card installments:

To charge a card in monthly installments instead of a single payment, pass an
`installment_plan_id` obtained from `list-installment-plans`. The `amount` and `currency`
must match the values you passed to `list-installment-plans`, and the card you tokenize
must be the card whose BIN (or token) you used there, so the chosen plan stays eligible
and the customer's quote remains consistent. Omit `installment_plan_id` to charge the card
as a one-time payment.

After starting the payment flow, you should poll `discover-next-action` to track the payment status
and determine the next steps.

## Endpoint signature

```http
POST https://api.reservepay.com/sdk/select-payment-method HTTP/1.1
Content-Type: application/json
Accept: application/json

{
  merchant_id: string,
  installation_id: string,
  installment_plan_id: string?,
  amount: string,
  currency: string,
  payment_method: string,
  token: string?,
  customer_email: string?,
  customer_mobile: string?,
  customer_name: string?,
  customer_language: string?,
  interest_bearer: string?,
  street_address_1: string?,
  street_address_2: string?,
  postal_code: string?,
  city: string?,
  state: string?,
  country: string?,
}
```

Returns: string

## Request arguments

### `merchant_id` (string)

**Required**. Your merchant ID that uniquely identifies your account.

### `installation_id` (string)

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

### `installment_plan_id` (string)

_Optional_. For card installment payments only: the plan ID chosen from `list-installment-plans`. Omit for one-time card payments.

### `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.

### `payment_method` (string)

**Required**. The payment method code (e.g. CARD, PROMPTPAY, K_PLUS). Must be enabled for your account.

### `token` (string)

_Optional_. For card payments only: The card token obtained from `tokenize-card`. Required when payment_method is CARD.

### `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.

### `customer_language` (string)

_Optional_. Customer's preferred language for communications. Defaults to `en` when omitted.

Allowed values:

- `en`
- `th`

### `interest_bearer` (string)

_Optional_. Who bears the installment interest: `customer` or `merchant`. When omitted, the merchant's configured default is used; if the merchant has no default configured this argument is required. Must match the value passed to `list-installment-plans` so the customer's displayed quote matches the locked snapshot. Ignored when `installment_plan_id` is omitted.

Allowed values:

- `customer`
- `merchant`

### `street_address_1` (string)

_Optional_. The first line of the address (e.g. street number and name).

### `street_address_2` (string)

_Optional_. The second line of the address (e.g. apartment, suite, or unit).

### `postal_code` (string)

_Optional_. The postal or ZIP code of the address.

### `city` (string)

_Optional_. The city of the address.

### `state` (string)

_Optional_. The state, province, or region of the address.

### `country` (string)

_Optional_. The country of the address.

## Errors specific to this endpoint

### `INVALID_ARGUMENTS`

This error occurs when the request contains invalid or missing parameters. Common cases include:

- A missing `merchant_id`, `installation_id`, `amount`, `currency`, or `payment_method`
- An unsupported `currency` (must match the funding currency configured for your provider)
- An installation that could not be found or does not have the requested payment method enabled
- Missing customer email or mobile when required by the installation
- Missing address fields when required by the installation and payment method

### `NOT_FOUND`

This error occurs when we cannot find the merchant account, installation, or card token with the IDs provided. Double check that you are using the correct IDs.

### `TOKEN_ALREADY_USED`

This token has already been used to create a payment session. Each token can only be used once. Request a new token from `tokenize-card`.

### `NOT_ENABLED`

This installation is currently disabled.

### `INSTALLMENT_NOT_ELIGIBLE`

The installment plan you provided is no longer available for this amount/card combination. Re-fetch eligible plans via `list-installment-plans` and try again.

## 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 is either deprecated or not yet released. This commonly happens when using an outdated SDK or when the API version specified in the request URL is incorrect.