Hi just_joelking,
We have more information about renting a car in Costa Rica on our website. This page (https://www.twoweeksincostarica.com/costa-rica-rental-car-discount/) answers frequently asked questions about insurance, if 4×4 is needed, etc. As it says on the page, we actually get a discount through one of the companies for our clients so you might want to check that out too.
The short answer, though, is basic liability insurance is included for all car rentals in Costa Rica and is government mandated. It’s usually referred to as “loss damage waiver” and is a daily line item charge. This is minimal basic liability protection- it doesn’t cover much, but you’re right, that most people are also covered through their credit card company for much more (You may want to double check about coverage through your US auto policy because usually they don’t cover international travel). We’ve always declined the additional collision damage waiver after checking with our US credit card company that we were covered but have still had to pay the mandatory insurance. I can see Expedia and others not automatically including it when you reserve online (some car companies in CR don’t include it in the initial rental estimate either), but you will have to pay it no matter what, so be sure to budget for it when comparing companies.