My car is totalled loss,do i have to accept their 1st offer?

Generally all the insurance company has to do is pay market value. Add ons and modifications are seldom considered in the value equation. You should do a market survey yourself to determine whether the insurance company is accurately appraising the car. Also, are you injured? - make a claim if you are hurt. Never trust the insurance company - they just want to get off the hook for the cheapest costs.