Return value in function/trigger

I have created a trigger and the function executed by the trigger. 
However the function merely updates values in a table and doesn't return 
any result/value. what is the correct way of "returning" nothing?

I have written this but using "return new" seems pointless as I don't 
need(?) to return anything.

create or replace function update_invoice_price() returns opaque as '
   begin
     update invoices set total_price=(select sum(price) from invoice_li 
where invoice_id=new.invoice_id) where id=new.invoice_id;
     return new;
   end;
' language 'plpgsql' with (iscachable);

create trigger update_invoice_li_price after update
   on invoice_li for each row
   execute procedure update_invoice_price();

Thanks!

Jc