You can use the time tracked for your employees to invoice labour to your customer. This will require that you have set the labour rate for employees correctly in the Settings >> Employees menu (refer to resource em1). There are various methods to do this as follows (from the most simple to more complex scenarios)
You can perform a simple transfer by clicking on Transfer timesheet to invoice. 2 options will be presented: "transfer as 1 line" and "transfer as 1 line per item". The former will work out the total hours and total amount to be invoiced, then create 1 invoice line item called Labour with the corresponding quantity and rate. The latter will simply carbon copy the timesheet section on to the invoice.
TRANSFER AS 1 LINE
TRANSFER AS 1 LINE PER ITEM