I just came from vacationing in Guanacaste Costa Rica and I used Serivicos Rizo transportation several times during my stay at Flamingo Beach Resort. They have a post right there at the hotel and other hotels as well (Westin) and they are 100% reliable. Their vehicles are pretty modern with the much-needed air conditioning and very friendly staff.
I used them for short trips as well as a long haul from Cañas when coming from Arenal Volcano. They will do one-way and round-trip fares for you and they are pretty moderately priced for a very comfortable ride. Some drivers may even suggest where to do what (restaurants, super markets, beaches etc...)
I was completely satisfied with their services.