In certain circumstances you want to add GST for just one of the items e.g. Mileage. You can do so by choosing to Invoice Tax Exclusive
On next page, tax will be calculated on for all line-items. However, you untick tax from all line-items except Mileage (for our example). See below, tax is ticked for Mileage line-item only and it has been unticked for other line-items.
This will ensure that tax is appropriately calculated for Mileage only.