I'm not sure about that...the slip has a number on it that they'll eventually call out over the PA system, but I guess you could try to get whatever information you can from whoever confirmed that it is here and go with that info and your passport to the international post office to see if they'll help. Take a book. Hopefully you're Spanish is good enough because they bark out the numbers in groups without repetition. Also, in case you don't already have it, the office is located here:
Centro Postal Internacional
Letonia y Antartida
Horario: 10:00 a 17:00 HS.
de Lunes a Viernes
Suerte