Oracle SQL Conditional SUM and TO_CHAR

I have a table with a sum that I want to format with $ at the beginning, but it gives me an error:

ORA-01722: invalid number 01722. 00000 - "invalid number" *Cause: The specified number was invalid. *Action: Specify a valid number.

select sum(TO_CHAR(a.freight, '$99,990.00')) AS "Costo total", (b.company_name) AS "Compañía fletera"
FROM orders a
inner join shippers b
on a.ship_via = b.shipper_id
group by b.company_name
order by b.company_name asc;

what I want is to see "Costo total" with a money format like $123.45