Getting rental car in Costa Rica is very tricky. It is compulsory in Costa Rica to get 3rd party insurance. Liability insurance (damages of our own car) is optional, but it is highly recommended to have atleast partial coverage of this. Liability coverage from US credit cards will work, but you need to get written approval from the credit card company for it to be accepted by the rental agency.
The common rental car sites of US like Enterprise, Hertz, Dollar and other travel sites like Expedia, Booking show insanely cheap prices in their site. But they may add extra prices like liability insurance, 3rd party insurance etc on top of your price. The price quoted by these big firms won’t be the final price, and your final price will eventually become much more than what you had anticipated. At this point, you don’t have much time to look for other options, and would need to accept whatever they quote.
During our trip, we took rental car from Car Rentals Costa Rica and USA (https://www.facebook.com/CarRentalsCostaRicaAndUSA/). This is owned by Jairo. Found him after many researches in Youtube/reddit/blogs, and this guy is totally reliable. All the quotes are informed upfront, and there won’t be any extras on top of what is quoted. Jairo will also deliver the car to your place (either airport or hotel) without any extra fees. Inform clearly the pick up time and location. They have services in San Jose, Liberia and all over Costa Rica.
During our trip, I came across another person whose flight landed 2 hrs late and the rental agency hiked the rental car price by $200 for late arrival. Keeping all this in mind, I highly recommend Jairo for the car rental.