After doing extensive research I have found that Adobe has best rates and great service, with offices through out the country should you encounter any problems. I would recommend a larger 4WD just for the comfort it affords. Click on the link to make your reservation. Adobe Car Rental
The rates include all taxes and the mandatory insurance ($1,100 deductible) your credit card should offer an insurance policy that will cover any costs above the deductible. FYI- There are no car rental companies in Costa Rica that will accept cash or a debit card for either payment or security deposit.
They also offer cell phone rental for my guests - $10 for the duration of the rental– free local calls; able to receive international calls and you can make international calls with a calling card. The phone must be reserved at the time the car is reserved, you need to request the phone in the comments box.
If you go on line to get quotes from other car rental agencies make sure they include in the rate the cost of insurance and taxes, most companies don’t do this and when you arrive you find out the actual cost is much higher than you were quoted.
Please let us know if you rented a car so that we can monitor the reservation.