Lots of cars make it easy to change out bulbs. But some cars are a massive pain to change certain bulbs, and/or require special tools. Do you exclude them? Or upcharge them? (E.g. the left headlight bulb on a Subaru Outback is an absolute nightmare.)

Unless you are doing this purely as a public service, $10 seems VERY low. Many independent mechanics in my area charge a minimum of 1 hour of work for anything they do, and bill at $90/hr and up. And they won't come to you for that price. (Nor are mechanics a highly paid profession in general.)

I really do wish you all the best, maybe I'm just not seeing the whole picture.

Edit: Something else to think about: when I replace bulbs in my cars, I always do it in pairs. Reason being that if one side burned out, the other is likely not far behind, especially headlight bulbs. The headlights also have another quirk: if you replace one and not the other, you usually end up with the new one being much brighter than the old one.