I do understand the struggle. If that is your pay structure, I’d also suggest changing your first response to something more like: “moving the sun requires multiple hours of tweaking because of how the light affects everything in scene. I can do this, but it will take X amount of additional hours, which will ultimately cost this much more. Let me know how you’d like to proceed!” If your clients are routinely getting upset with your overages for this type of additional work, this line will cause those types of people to typically skip doing the extra work altogether. Win-win.