Update

//  Create a Map to store all the data of an Estimate
Map<String, Object> nsEstimate = new Map<String, Object>();
    nsEstimate.put('internalId', '38310');
    nsEstimate.put('entityStatus', new Map<String, Object>{'internalId'=>'11'});
    nsEstimate.put('expectedCloseDate', DateTime.newInstance(2020, 10, 31).getTime()/1000);

    //  Initializing the Estimate Line Items
    List<Object> lineItemList = new List<Object>();
    Map<String, Object> lineItem = new Map<String, Object>();
        lineItem.put('line', 1); // updates the existing line item with line number as 1.
        lineItem.put('quantity', 2);
        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 Estimate
    Map<String, Object> estimateLineitemList = new Map<String, Object>();
        estimateLineitemList.put('item', lineItemList);
        estimateLineitemList.put('replaceAll', false); // setting this as false, would create a new, or update, the line items passed in the Request based on the Line Number.

    nsEstimate.put('itemList', estimateLineitemList);

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

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

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