Create Invoice

Map<String, Object> invoiceData = new Map<String, Object>();
invoiceData.put('processorCustomerId', 'processorCustomerId');  
invoiceData.put('invoiceStatus', 'SEND_TO_CUSTOMER'); 
invoiceData.put('daysUntilDue', 2); 

List<object> items = new List<object>();
Map<String, Object> itemData = new Map<String, Object>();
itemData.put('processorPriceId', 'processorPriceId'); 
itemData.put('quantity', 2); 
items.add(itemData);
invoiceData.put('items', items); 

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

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

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