Guatemalan application for a Z type visa

Dear,
I have read the answer from Julia (Ireland). "The first thing is to prepare the required documents including the passport and copy of its data page, the completed application form attached with a passport-sized color photo, the CR residency permit, work permit and an invitation letter from a duly authorized unit in PRC. Your future employer will get you the work permit and invitation letter. "
My company is stuck at the invitation letter part, who extends it?
I got the work license already and now they dont know if the Embassy in Costa Rica should extend this invitation letter and how to apply from the Z visa from PRC.
My residency permit should be Guatemalan? Normally obtained at my city hall?
If there is any email where to get in touch with the Costa Rican Embassy please let me know, the one above isnt working, I'm getting fatal errors from it.
Muchas gracias,
Carlos