Apple Bridgeport Village

I purchased a MacBook air on December 2014, I've been using the computer for work for about years, last 11/11/after a system update the computer didn't start back on (I have a video from a surveillance camera as proof that It was working fine before the update) I talked to apple support, I did everything they told me to and still nothing, I took it to store for repair but they couldn't find the problem, the warranty they offer is only for year so it was not covered anymore, so they offer me a flat rate of $for the repair, same that I agreed because I needed the computer, When I picked it up I asked the person who helped me to send me a detail invoice with the cause of the failure since they did not find any water or fiscal damage in the computer, It's always plug in a surge protector, they never did, the invoice only states the parts they replaced and charges but not the cause of failure, My business insurance will not cover the repair and Income loss unless they that