ORA-04091:table is mutating, trigger/function may not see it

hi expert .
I want to write a trigger.
I want him to throw an error when the 'ucreti' column exceeds 500.
trigger is running in insert operation.
But Trigger is failing to update.
for example

CREATE PACKAGE ucret_islemyap IS
  ucretler Number;
END;
create table islemyap
(
rowid1 varchar2(20),
protokol_no number,
dosya_no number,
ucreti number,
fatura_tipi number
)
Insert into HASTANE.ISLEMYAP
   (ROWID1, PROTOKOL_NO, DOSYA_NO, UCRETI,fatura_tipi)
 Values
   ('A198211', 838, 725, 70,1);
Insert into HASTANE.ISLEMYAP
   (ROWID1, PROTOKOL_NO, DOSYA_NO, UCRETI)
 Values
   ('A198212', 838, 725, 22);

Select allOpen in new window

I wrote trigger as below

create or replace trigger burak_islem_ucreti
for insert or update on islemyap compound trigger
toplam number;
before each row  is 
  begin

        if :new.FATURA_TIPI in (1,3) then
        begin
           
              select sum(iy.ucreti) into ucret_islemyap.ucretler  from islemyap iy  where IY.PROTOKOL_NO=:new.protokol_no and IY.FATURA_TIPI in (1,3);
              toplam:=ucret_islemyap.ucretler+nvl(:new.ucreti,0);
                if toplam>=500 then
                        begin
                        raise_application_error(-20101, 'fiyatiniz 500 tl yi asmistir');
                        end;
                end if;
          end;
      end if;
      
end  before each row ;
end;

Select allOpen in new window

running for trigger insert operation.
but trigger update doesn't work
for example

update islemyap iy 
set 
UCRETI=700
where IY.ROWID1='A198211'

Select allOpen in new window

trigger gives me error

ORA-04091: table HASTANE.ISLEMYAP is mutating, trigger/function may not see it

Select allOpen in new window

how can I solve this problem?
thanks a lot .