We have not had the time to implement "flat rate projects" yet, but that is on our ToDo list. What we have been recommending to people right now is to do the billing using the Expenses. Bill your client for the project using one expense entry. Next add yourself as a Client. Now when you create Time Entries, bill them to yourself. That way you can track the time to know how well you are doing on estimates, but still be able to bill the flat rate to your client using the Expenses.