I assume you mean the Amex travel insurance policy that also includes car hire? This policu does not include local taxes, surcharges and the like. This works out to about $70 a week if my memory serves me. Decline Collision Damage Waiver and Supplementary Liability Insurance, as these are included on the Amex policy.
You wil probably be asked to leave a deposit of $1,500 if you decline the rental company's own insurance. Simply leave a credit card imprint for this, although it is probably a good idea to tell your credit card company about this before you travel to make sure they will authorise it.