trigger makeInvoice on Opportunity (after insert, after update) {
   
    for(Opportunity op :trigger.new){
        if(op.StageName == 'Closed Won - Signed'){
           List<OpportunityLineItem> opli = new List<OpportunityLineItem>();
            opli = [select id,Gross__c, Product2id, OpportunityId from OpportunityLineItem where OpportunityId =: op.id order by Product2id];
            if(opli.size() > 0){
                String tmpId = '';
                List<Invoices__c> lstInvoice = new List<Invoices__c>();
                Invoices__c objInvoice = new Invoices__c();
                Integer lm = opli.size();
                Integer i = 1;
                for(OpportunityLineItem ob: opli){
                    if(tmpId == ''){
                        tmpId = ob.Product2id;
                        objInvoice = new Invoices__c();   
                        objInvoice.Opportunity_invoices__c = ob.OpportunityId;
                        objInvoice.Amount__c = ob.Gross__c;                                      
                    }else{
                        if(tmpId == ob.Product2id){
                            objInvoice.Amount__c = objInvoice.Amount__c + ob.Gross__c;
                        }else{
                            lstInvoice.add(objInvoice);
                            tmpId = ob.Product2id;
                            objInvoice = new Invoices__c();   
                            objInvoice.Opportunity_invoices__c = ob.OpportunityId;
                            objInvoice.Amount__c = ob.Gross__c;                           
                        }     
                    }
                    if(i == lm){
                        lstInvoice.add(objInvoice);
                    }
                    i++;
                }
                if(lstInvoice.size() > 0){
                    try{
                        insert lstInvoice;
                        system.debug('!Lst '+lstInvoice);
                    }catch(Exception e){
                        system.debug('!Bingo '+e);
                    }
                }
            }
        }
    }
}