begin
  insert into Заказы ...
excption when DUP_VAL_ON_INDEX then -- unique key (заказИД, товарИД)
  update ...
end;

merge 

i number;

update HEAD set STATUS = 1 where ID = p_id_head;
for S in (select PRODUCTID, QTY from SPC where ID_HEAD = p_id_head) loop
   select ID_PRODUCT into i from PRODUCT_LOCK where PRODUCTID = S.PRODUCTID for update;
   merge into PRODUCTS P ... using ( ... ) NEWP on (P.PRODUCTID = NEWP.PRODUCTID)  when not matched then insert ... when matched then update set QTY = QTY - NEWP.QTY;
end loop;

   merge into PRODUCTS ...