It's definitely ok, and it will probably work out better for you than asking a higher hourly wage. The client may of course be reluctant to accept this if they haven't done any similar arrangements before.
As a side note, having flexible hours is great if you need that kind of flexibility in your life. If you don't, consider billing by day/week.
As a side note, having flexible hours is great if you need that kind of flexibility in your life. If you don't, consider billing by day/week.