If you can wait and got time, I would wait till the summer time and SunBright, it is a longer process but without the chemical.
Retr0bright & Sunbrighting are not guaranteed to last so I doubt there is a service for this
I've had mixed results from both, one amiga has stayed white so far the other the yellowing has returned I've also had it go wrong where I've cause a marble effect on the case when Retr0brighting