Having taken into account users' requests, TO3000 V8 and newer versions include a way to add untaxed expenses to the Invoice.
Now expenses can be added on the Adjustments tab of the New/Edit Invoice window. The Adjustment sum (Volume field value) is considered in the Invoice Total, but taxes and discounts are not applied to it.
You may use the following variables in your template:
\ADJUSTMENTS_VALUE\ - Adjustments volume
\ADJUSTMENTS_DESCR\ - Adjustments description
\BEFORE_ADJUSTMENTS\ - The sum of all jobs cost with discounts and taxes applied (i.e. Invoice Total without Adjustments).
\INVOICE_HAS_ADJUSTMENTS\ - Boolean variable. Its value is "True" if there were Adjustments added to the Invoice and False if there were no Adjustments added.