The steps to integrate with BNI Virtual Account are given below.

Send the charge API request to Midtrans.
Display the virtual account number.
Handle notifications.

Send a Charge API request with the details of the transaction such as payment_type, bank_transfer, transaction_details, item_details, and customer_details. Successful request returns a VA number.

By default, default expiry time for bank transfer is 24 hours unless specified by merchant (min 20s, max 180 days).

BNI Charge API Request

{
    "payment_type": "bank_transfer",
    "transaction_details": {
        "gross_amount": 10000,
        "order_id": "{{$timestamp}}"
    },
    "customer_details": {
        "email": "budi.utomo@Midtrans.com",
        "first_name": "budi",
        "last_name": "utomo",
        "phone": "+6281 1234 1234"
    },
    "item_details": [
    {
       "id": "1388998298204",
       "price": 5000,
       "quantity": 1,
       "name": "Ayam Zozozo"
    },
    {
       "id": "1388998298205",
       "price": 5000,
       "quantity": 1,
       "name": "Ayam Xoxoxo"
    }
   ],
   "bank_transfer":{
     "bank": "bni",
     "va_number": "111111"
  }
}

JSON Attribute	Description	Type	Required
payment_type	Set Bank Transfer payment method. Value: `bank_transfer`	String	Required
transaction_details	The details of the specific transaction such as `order_id` and `gross_amount`.	Object	Required
customer_details	Details of the customer.	Object	Optional
item_details	Details of the item(s) purchased by the customer.	Object	Optional
bank_transfer	Charge details using bank transfer.	Object	Required

BNI Virtual Account Sample Responses and Notifications

{
  "status_code": "201",
  "status_message": "Success, Bank Transfer transaction is created",
  "transaction_id": "9aed5972-5b6a-401e-894b-a32c91ed1a3a",
  "order_id": "1466323342",
  "gross_amount": "20000.00",
  "payment_type": "bank_transfer",
  "transaction_time": "2016-06-19 15:02:22",
  "transaction_status": "pending",
  "va_numbers": [
    {
      "bank": "bni",
      "va_number": "8578000000111111"
    }
  ],
  "fraud_status": "accept",
  "currency": "IDR"
}

{
  "va_numbers": [
    {
      "bank": "bni",
      "va_number": "8578000000111111"
    }
  ],
  "transaction_time": "2016-06-19 15:02:22",
  "gross_amount": "20000.00",
  "order_id": "1466323342",
  "payment_type": "bank_transfer",
  "signature_key": "74d9b6f4cc36585d74e06c9dba360331e455171bcec35df60cf28a786436f8f96e6afcc1f091d6cb61411aec246ac28ba30b76d9b2c1cdb6409c0a70fcc1fe47",
  "status_code": "201",
  "transaction_id": "a9aed5972-5b6a-401e-894b-a32c91ed1a3a",
  "transaction_status": "pending",
  "fraud_status": "accept",
  "status_message": "midtrans payment notification"
}

{
  "va_numbers": [
    {
      "bank": "bni",
      "va_number": "8578000000111111"
    }
  ],
  "payment_amounts": [
    {
      "paid_at": "2016-06-19 20:12:22",
      "amount": "20000.00"
    }
  ],
  "transaction_time": "2016-06-19 19:12:22",
  "gross_amount": "20000.00",
  "order_id": "1466323342",
  "payment_type": "bank_transfer",
  "signature_key": "fe5f725ea770c451017e9d6300af72b830a668d2f7d5da9b778ec2c4f9177efe5127d492d9ddfbcf6806ea5cd7dc1a7337c674d6139026b28f49ad0ea1ce5107",
  "status_code": "200",
  "transaction_id": "9aed5972-5b6a-401e-894b-a32c91ed1a3a",
  "transaction_status": "settlement",
  "fraud_status": "accept",
  "status_message": "midtrans payment notification"
}

{
  "status_code": "202",
  "status_message": "midtrans payment notification",
  "transaction_id": "9aed5972-5b6a-401e-894b-a32c91ed1a3a",
  "order_id": "1466323342",
  "gross_amount": "20000.00",
  "payment_type": "bank_transfer",
  "transaction_time": "2016-06-20 15:02:23",
  "transaction_status": "expire",
  "signature_key": "e4e829a5d9a1e342daf181c5fd750afa64dd5a5c9a6e0cfb636e7966295b2613fda22b4dc52b01c30618a50ff3524f57ec661a9dced249df93f163138a0df6b0"
}

JSON Attribute	Description	Type
status_code	Status code of transaction charge result.	String
status_message	Description of transaction charge result.	String
transaction_id	Transaction ID given by Midtrans.	String
order_id	Order ID specified by you.	String
gross_amount	Total amount of transaction in IDR.	String
payment_type	Transaction payment method.	String
transaction_time	Timestamp of transaction in ISO 8601 format. Time Zone: GMT+7.	String
transaction_status	Status of bank transfer transaction. Value: `pending` : Bank Transfer transaction is created.	String
fraud_status	Detection result by Fraud Detection System (FDS). Value: `accept` : Approved by FDS.	String
va_numbers	Bank name and Virtual Account number generated by Midtrans.	JSON Array
payment_amounts	Payment histories for the transaction. RECOMMEND TO NOT USE payment_amounts legacy field as payment_amounts is only filled in BNI-VA and is set as empty array [] in other banks (BCA, BRI, Permata) RECOMMEND TO USE gross_amount field field instead since gross_amount field will always be there for all banks VA (BNI, BCA, BRI, Permata).	JSON Array
currency	ISO-4217 representation of three-letter alphabetic currency code. Value: `IDR`. Note: Currently only `IDR` is supported.	String
signature_key	Combination of parameters such as `order_id`, `status_code`, `gross_amount`, `server_key` to verify integrity of payload/response.	String

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Note
Possible error codes are 400, 401, 402, 406, 410