Update

//Create a Map to store all the data of a Invoice
Map<String, Object> nsInvoice = new Map<String, Object>();
	nsInvoice.put('internalId', '91404');
    nsInvoice.put('classification', new Map<String, Object>{'internalId'=>'2'});
    nsInvoice.put('department', new Map<String, Object>{'internalId'=>'1'});
	nsInvoice.put('memo', 'Invoice 215');
    nsInvoice.put('otherRefNum', '4343400');
    nsInvoice.put('tranDate', DateTime.newInstance(2020, 10, 07).getTime()/1000);
//  Initializing the Invoice Custom Fields
    List<Object> invCustomFieldList = new List<Object>();
    Map<String,Object> invCustomField = new Map<String,Object>();
        invCustomField.put('fieldType', 'boolean');
        invCustomField.put('scriptId', 'custbody_checkbox');
        invCustomField.put('value', 'true');
        invCustomFieldList.add(invCustomField);

//  Adding the Invoice Custom Fields
    nsInvoice.put('customFieldList', new Map<String,Object>{'customField'=>invCustomFieldList});

    //  Initializing the Invoice Line Items
    List<Object> lineItemList = new List<Object>();
    Map<String, Object> lineItem = new Map<String, Object>();
        lineItem.put('description', ' For shooting arrows');
        lineItem.put('line', 1); // updates the existing line item with line number as 1.
        lineItem.put('quantity', 6);
		lineItem.put('revRecEndDate', DateTime.newInstance(2021, 09, 07).getTime()/1000);
        lineItem.put('revRecStartDate', DateTime.newInstance(2021, 09, 07).getTime()/1000);
//  Initializing the Line Item custom fields
        List<Object> liCustomFieldList = new List<Object>();
        Map<String, Object> liCustomField = new Map<String, Object>();
            liCustomField.put('fieldType', 'boolean');
            liCustomField.put('scriptId', 'custcol_checkbox');
            liCustomField.put('value', 'false');
            liCustomFieldList.add(liCustomField);

//  Adding the Invoice Line Item Custom Fields
        lineItem.put('customFieldList', new Map<String, Object>{'customField'=>liCustomFieldList});
        lineItemList.add(lineItem);
//  Adding Another Line Item
	lineItem = new Map<String, Object>();
        lineItem.put('item', new Map<String, Object>{'internalId'=>'444'});
        lineItem.put('quantity', 4);
        lineItem.put('price', new Map<String, Object>{'internalId'=>'2'});
        lineItemList.add(lineItem);

//  Adding the Line Items to Invoice
    Map<String, Object> invLineitemList = new Map<String, Object>();
        invLineitemList.put('item', lineItemList);
		invLineitemList.put('replaceAll', false);
    nsInvoice.put('itemList', invLineitemList);

//  Initializing the RequestJSON Data to be passed.
Map<String, Object> requestJSONMap = new Map<String, Object>();
    requestJSONMap.put('invoices', new List<Object>{nsInvoice});
    String reqJSON = JSON.serialize(requestJSONMap);

Map<String, Object> reqObj = new Map<String, Object>();
    reqObj.put('version', '1.0');
    reqObj.put('action', 'updateInvoice');
    reqObj.put('requestJSON', reqJSON);

//  Placing a request to Breadwinner Global API
Map<String, Object> resp = breadwinner_ns.BreadwinnerNetSuiteAPI.call(reqObj);
System.debug(resp);