Can a friend in Costa Rica do the application for me?

Dear Sirs.
I live in Guatemala City and there is a possibility that I need to attend a meeting in Beijing next April 2013. I want to know if I need to travel to Costa Rica and go personally to your Embassy to apply for a visa or if a friend of mine (who lives there) can do the process for me (delivering all the documents you request).
Thank you for your time!