If I am only booking a flight, I buy directly from the airline. In case I want to change or cancel a flight (it happens), I only have to pay the airline change fee. Last I checked, Orbits and Expedia (and AMEX travel) charge their own change fee in addition to the airline.

I also look at the one-way flight schedules, to get the times I want. When I search round trip, sometimes only a couple combinations come up for choice.