Custom Order With Sound Design

Four days ago, I got a message from someone on Fiverr. They sent me a short animation with a separate script. They needed me to voice a young ten-year-old boy, and he had 37 words. My prices on Fiverr are $10 for every 50 words, so the amount in all would be $10. On my gig, you can order extra things like proofreading, background music, one-day delivery, etc. They asked if the music included sound design (adding sound effects). I told them it doesn't, but that I do sound design, but I don't advertise it on Fiverr. I said I'd be willing to help them out with it. So I sent them the lines for the voice-over part, and now we're figuring out the audio engineering part. I want to charge them $40 for the music and the sound design.