How to add a 'total' row in a grouped query (in Postgresql)?

The rollup function in this https://stackoverflow.com/a/54913166/1666637 answer might be a convenient way to do this. More on this and related functions here: https://www.postgresql.org/docs/devel/queries-table-expressions.html#QUERIES-GROUPING-SETS

Something like this, untested code

WITH w as (
    --SELECT statement from above--
)
SELECT * FROM w ROLLUP((money,requests))

Note the double parentheses, they are significant

You can achieve this by using a UNION query. In the query below, I add an artificial sortorder column and wrap the union query in an outer query so that the sum line appears at the bottom.

[I'm assuming you'll be adding your joins and group by clauses...]

SELECT section, money, requests FROM  -- outer select, to get the sorting right.

(    SELECT 
        organizations.name || ' - ' || section.name as Section, 
        SUM(requests.money) as money, 
        COUNT(*) as requests,
        0 AS sortorder -- added a sortorder column
     FROM 
        schema.organizations
    INNER JOINs omitted --
    WHERE 
        -- omitted --
    GROUP BY 
        -- omitted --
       --  ORDER BY is not used here


UNION

    SELECT
       'BMO - Total' as section,
        SUM(requests.money) as money, 
        COUNT(*) as requests,
        1 AS sortorder
    FROM 
        schema.organizations
        -- add inner joins and where clauses as before
) AS unionquery

ORDER BY sortorder -- could also add other columns to sort here