Is additional insurance required for rental cars in Costa Rica?

Is it required to purchase insurance from the rental car company? My understanding is that certain credit cards act as your primary insurance if you use it to pay for the car, and your auto insurance policy in the US covers liabilities to other drivers (If you have full coverage).  We’ve read that even though you may waive the collision damage waiver when you order from sites like Expedia, the company will still add on additional fees upon arrival in Costa Rica.  Is that true?  We haven’t been required to purchase anything additional in other countries.