Dividing by quality "Good, Great, and Excellent" is weird to me. What metric distinguishes the different options? If I only check "Good," will you edit my text while watching television instead of devoting your full attention to it?

I would get rid of the quality gradient altogether, instead ensuring your users that all of your work is "excellent." If you still want an extra tier to your pricing, I guess you could do what translation services do and charge per revision or by number of people the text passes through, but the idea that you simply aren't trying as hard if I check "good" doesn't sit well.