s3.4.12(1)(a) exempts a practitioner from disclosure if the total legal costs in a matter, excluding disbursements, are not likely to exceed $750.00. If the charge is over $750.00 (exc. GST) then you must provide disclosure.
If the charge to be made is not to be calculated on scale, and you do not want your costs assessed on scale, in the event of a dispute, then you must have a costs agreement.