I prefer to work fixed price. We agree on the deliverables and I calculate a price for it. A topic list and many years of experience enable me to calculate the required labour quite accurately. It is the best solution for you as a client, as you know exactly what you will get at what cost. It keeps me sharp and it forces me to work efficiently since any overtime is on my account.
Do you prefer to work on a time & materials base nevertheless? That’s fine with me. I keep a record of my work hours and I send you a time sheet at the end of each month. I invoice you upon approval of the time sheet.