Upon execution of the agreement, we set up a project for each sprint in our Harvest Time Management System. This allows us to parallelize work while attributing it to a specific deliverable. Sprints represent a number of man-hours, but multiple individuals might be involved in the time & effort. Upon commencement of a Sprint, we send an invoice for the stated amount. That's a bank of hours from which we draw against to deliver the integration of our solution. If there's an overage above 10% of the estimate T&E, we'll prepare a detailed/itemized invoice against that sprint and bill for the excess hours. This is all handled automatically by our bookkeeper.