According to the study abroad newsletter we received last May in Salamanca:
"Airmail your packages home through Correos. Your package will take a minimum of 3 wks to arrive in the US. You must wrap your package and they prefer it to be in a box (not just your suitcase). Price for mailing from 1 to 20 kilos is 1.574 ptas. per kilo. Maximum package measurements do exist.
Shipping your package home costs less but be prepared to wait 3 months! 20 kilos (max weight)= 9750 ptas., 10 kilos = 5134 ptas, 5 kilos = 2925 ptas."
I recall friends who shipped stuff home getting a cheaper rate if they said the package contained books.