www.CostaRicaTicas.com

It will let you off at a terminal downtown - you just return there to get the bus back to the airport - you should verify the return times for the day and time you want to leave.

Also - get the name of the terminal, and the "Tico directions" for the place (your driver will know what you mean - almost no one uses addresses in Costa Rica....) so you can give it to a taxi driver for your cab back to the terminal on the day you are leaving - I think the bus runs frequently but it pays to verify.