for that, what should i do? I do interior repairs for used car lots. One of them has asked for a certificate of liability insurance, but there isnt any need for it. I dont move the cars and just fix seats, etc. Is there some kind of waiver that i can give them which says that they are not liable of any injuries that i may incur and that i am liable for all damages?

This is business liability insurance and not automobile liability insurance. It's probably about $300/year for a mill. It's well worth it because if you damage something, your client goes after the insurance and not you and your business. Plus it gets them off your back for asking about it.

It's a busines expense and can be deducted from your taxes .....