CTE Query copy value if exists into an existing table
What you have so far can be rewritten to this 100 % equivalent query:

INSERT INTO parts
        (description, item_code, created,   updated,   sale_p     ,   price, code_p_id, s_id_code)
SELECT t.description, t.code , t.created, t.updated, t.sales_price, t.price, rp.id    , rc.id
FROM  (
   VALUES 
     ('Small Bus', '1dfsdf4', now(), now(), 0.15 , 0.51 , 'Tax0'      , '242')
   , ('Window'   , 'QSF-312', now(), now(), 350.0, 550.0, 'cdc'       , 'ded')
   , ('Test 1'   , 'EXD-TUH', now(), now(), 5.0  , 10.0 , '54289hijlj', 'fjDIashdukah')
   ) t(description, code, updated, created, sales_price, price, code_p, sales_c)
JOIN   parts       p ON pa.item_code = t.code  -- why?
LEFT   JOIN rates rp ON rp.code      = t.code_p
LEFT   JOIN rates rc ON rc.code      = t.sales_c
WHERE  p.company_id = 65312322                 -- why?
RETURNING *;


The query already inserts NULL into code_p_id and s_id_code respectively if no matching row in rates is found.

You don't need CTEs here.
Be aware that the RETURNING clause returns actual resulting values of new rows after applying any rules or triggers there might be.

You description is still unclear.
The join to parts seems highly suspicious. parts.company_id is not defined UNIQUE, the join will probably multiply rows. I seriously doubt that's intended.

You might really mean to UPDATE selected rows in table parts, not INSERT new rows ...