Renting cars is about the most frustrating thing in Costa Rica. It feels like a scam, but with some additional info I think the challenges can be avoided.
TPL [aka Supplemental Liability Insurance (SLI)] - There is no way around this. It's mandatory, regardless of what coverage your credit cards provide or other personal insurance you have. Most online rental agencies leave this number out of their prices, whether that's Expedia or directly from the big rental car company site (Budget, Alamo, etc.). They'll use some generalized version of 'additional local charges may apply'.
I used Booking dot com (branded Rentalcars dot com) and rented from Avis because they included TPL in the final price. I had to dig into the details to determine that. If your contract includes TPL, bring a hard copy of the invoice that indicates such. The only way to find out what the TPL charge would be if they are not included, would be to contact the local office of whatever company you've rented from and get the rates.
CDW (Collision Damage Waiver) - No rental car companies include the CDW in the fees you see online. Your credit cards may have coverage to take care of the CDW the local agents say is mandatory, but you need to bring a hard copy letter from your credit card company stating that you have coverage. You may get away with providing the credit card company's generic policy statement that says rental car insurance is included, but a letter is better. The letter should have the name of the credit card company on it. Local agents will tell you that one of the major credit card companies name's has to be there because this is the only additional insurance they accept.
Please note, personal 3rd party liability insurance, which should give you the ability to decline all rental car coverage was not applicable. Even if your coverage is from a company like Chubb (the biggest insurance company in the world), they will not accept it. It needs to be from a credit card company. See above.
I watched a couple get dinged for $300 in unexpected charges at the time of rental. What an awful way to star a vacation.
Side note. Rent a 4x4. The roads here get rough sometimes, so if you're going further than Tamarindo, like Nosara or Santa Teresa you might prefer something larger. You could get by with the cheapest 4x4s they offer (Begos, Terios), but going slightly larger to something like a Nissan X-Trail will make you more comfortable.