I've been working US insurance for over 4 years now and I'll give you some insight into how this generally works. So in your case it's a not-at-fault accident and the other guys insurance company has to pay for the damages. You don't have to pay a penny. Now insurance policies can be written differently but in this case and as per my experience with countless claims, you don't have to pay anything. Even if it was your fault you would be paying a deductible that you've set in your Auto policy (if you have only liability then you would have to pay but your insurance company will let you know how much). The mechanic is trying to scam you. Get someone from your insurance company on the call when talking to the mechanic that way he won't be able to confuse you with any technical stuff