Costa Rican Rental Car Insurance Explained (sort of)
Auto insurance in Costa Rica is a government monopoly of the INS (Instituto Nacional de Seguros). Liability insurance from the INS is mandated by law, but provides only minimal coverage. Rental agencies offer supplemental programs that cover deductibles and contingencies other than a simple traffic accident.

The cost of insurance can approach or exceed the cost of renting. There are three types
Required by Law—TPL, SLI, SLC or API—Liability Insurance from the INS is required regardless of your policies at home or the claims of your titanium credit card. The cost ranges from $US 7-20 per day depending on the vehicle. This insurace does not cover your rental car at all, only damages to other people, their cars, or property. Unless you purchase supplements, or have documented coverage from your credit card you are still responsible for all damages to the rental vehicle.
Optional—CDW or LDW—Collision Damage Waivers are a supplement underwritten by the rental company to reduce your liability for your rental car to the deductable amount ($US 1,000-3,000 depending on the company and vehicle) only in the case of an accident. The cost ranges from $US 12-20 per day depending on the company and vehicle. You are still responsible for all losses or damages in the case of negligence, vandalism, road damage, or theft.
This insurance may be replaced by your home auto insurance policy or credit card coverage. Check before you rent, and get it in writing. Some credit card companies have exclusions specifically against Latin American rentals. Some rental companies do not allow you to waive this insurance regardless of other coverage. They are noted in the rate table.
Optional—Zero Liability—Under the names of Full Coverage, Vandalism Supplement, Damage Supplement, and Theft Insurance these extra policies are intended to reduce your liability to zero when combined with SLI and CDW. For example, if the trunk is damaged by thieves while they steal your luggage you will be held resposible for the damage unless you have purchased a third level of insurance. The cost ranges from $US 5-20 per day depending on the fine print and vehicle.
These insurances may be replaced by your home auto insurance policy or credit card coverage. Check before you leave home, and get it in writing. Some credit card companies have exclusions specifically against Latin American rentals.
More Confusion—The confusion and frustration involved in getting a fair price on a car rental gets worse before it gets better. Some rental car agencies lump the cost of one or more types of insurance into the rental cost, others combine the government insurance in their CDW or LDW coverage, and others seem to intentionally confuse the three types in order to make their rates appear better. We do not rent cars or have marketing agreements with any agencies and therefore have the same objective as you. We want to find the lowest rate from the most reliable company. The prices in the rental agency comparison represent the cost of the rental plus the minimum insurance required by law or the rental company. This would be the rate you would actually pay if your credit card covers the supplemental insurance, or you choose to assume the responsibility yourself. The rates in the table do not include the additional costs listed below.

Other costs of renting a car in Costa Rica.
Gas—Cars returned with less than a full tank of gas will be refuled at your expense. The price of gas from the rental companies can be up to twice the cost at a service station.
Government License Plate Fee— $US 1 per day, rarely included in quoted rates.
Airport Concourse Fee—There is a 12% airport concourse tax added to the cost of any car picked up at Juan Santamaría. The Car companies all know this and will make arrangements to help you avoid it. If you arrive late and plan to stay in the area (do not drive at night), take a taxi to your hotel and have the car delivered the next morning (often free of charge and saves you a day of rental). If you arrive early and want to get on the road, the car companies have off airport pickup locations a few minutes away.
Additional Drivers—$US 2.50-10 per day
Secure Parking—Recommended. Never leave anything in the car!
Ferry—Taking your car on ferry boats costs between $US 10-15.
Damages—Unless you purchase all of the above insurance or your credit card covers it, you are resposible for damage done by potholes, vandals, thieves, falling trees, high tide, army ants, spilled food, and any other imaginable occurence other than a traffic accident involving another party.
Tax—Finally you get a break. Car rentals are not taxed in Costa Rica. Companies that advertise prices with "All taxes included" are not lying, just missleading.