Salesforce ID's with comma seperated

First we have to create custom setting PaymentOpp and need to create field OppName along with values Mani .

public Calc(){
        //lstw =new List<wrapper>();
        oppidd = System.currentPagereference().getParameters().get('oppid');
        recid= new set<string>();
        String[] cd = oppidd.split(',');
        RowNo = cd.size();
        System.debug('RowNo--->'+RowNo );
        
     
    }
    


     String Finalval = '';
    string FinalvalSub ;

for(Opportunity Each:selectedOpportunities){
           oppid.add(each.id);
          
              
       }

for (iD Obj:oppid){
       
        Finalval = Finalval +','+ Obj;
       
          }
        System.debug('%%%%%%%Finalval '+Finalval );
        FinalvalSub = Finalval.substringAfter(',');
       
        //FinalvalSub = Finalval.substringBefore(',');
        System.debug('%%%%%%%FinalvalSub '+FinalvalSub );







List<PaymentOppId__c> PaySetting ;
       
      for(PaymentOppId__c PId:[Select Id,OppId__c, Name From PaymentOppId__c Where Name = 'Mani']){
         
          PId.OppId__c = FinalvalSub ;
          System.debug('FinalvalSub --->'+FinalvalSub);
          System.debug('PId--->'+PId);
          Update  PId;
              
      }

 if(Month>7){
        System.debug('Month#####'+Month);
        pg = new PageReference('/apex/RepaymentGenPage?CustName=Mani');
        //PageReference pg = new PageReference('/apex/PaymentSchedule?CustName=Mani');
       //PageReference pg = new PageReference('/apex/RepaymentGenReg?CustName=Mani');
      // pg.getParameters().put('oppid',selectedOpportunities[0].id);
       //String appid = ApexPages.currentPage().getParameters().get('Id');
       pg.getParameters().put('oppid',string.valueof(oppid));
       pg.setRedirect(true); 
       
       }
       
       
       else if( Month<=7) {
       
        //PageReference pg = new PageReference('/apex/RepaymentGenPage?CustName=Mani');
      
        pg = new PageReference('/apex/RepaymentGenReg?CustName=Mani');
       // pg.getParameters().put('oppid',selectedOpportunities[0].id);
        //String appid = ApexPages.currentPage().getParameters().get('Id');
        pg.getParameters().put('oppid',string.valueof(oppid));
        pg.setRedirect(true); 
          
       }
       return pg; 
       
    }