You need to be at SJO at least 2 hours prior to International Departure, Check In will close 1 hour prior flight. You need to allow an extra 1 hour for traffic/delays, and its a 3.5 hour drive. So you need to leave first thing in the morning, to be there by 11-11:30...
You can check with your lodging for what shuttles you can use and the schedule. Figure $55 PP for the trip.
Interbus & Grayline are most popular.
Depending what day Xmas falls on, if its a weekend, allow extra time for sure. I also don't know about shuttle services being normal for Xmas day either. Xmas eve could also be a issue, as most people there spend time with family and go to Church that afternoon evening.
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