Mutating Trigger...workaround needed

Sorry...not sure what is going on...this post keeps changing the code that I typed.  Ignore the rest of the post below the first section of blue.  Thanks!

I have an order table and a line item order table.  On the line item order table I have the following trigger

create or replace trigger order_total after insert or update or delete on mcis_line_item begin update mcis_order set order_total = (Select sum(decode(total_cost_override, null, unit_cost*quantity,total_cost_override)) from mcis_line_item where mcis_line_item.order_id = mcis_order.order_id); end;