FlexiTime invoices can be set up to have additional lines added based on other units (such as Hours) in the invoice, for example, if an admin fee of $2 / hour invoiced.
The left side shows the simple set-up for a Fixed charge at a rate of $2. The trick is getting the total of hours from the other lines in the invoice to use as the quantity. This is what Quantity Aggregator controls. When this is ticked, the quantities from other lines in the invoice with the same Units as this charge type (in this case Hours) are summed to get the quantity for this extra line.
The Sort Order has been set to Second to ensure that when the invoice is printed the admin fee appears after the other invoice lines (where the sort code on their charge types are set to First).
On the printed invoice, this admin charge will appear like this: