The system uses both 2 and 3 character country codes. 

The country code list is used to validate address information, and to interface address information to other systems. 
The country codes must be valid standard ISO country codes. See e.g. 
http://www.worldatlas.com/aatlas/ctycodes.htm

The country Shipping charge is normally not used, set to 0. (see Shipping Charge).