Upgrading SQL Server 2008 Standard to Enterprise
Have your client with the ISOs or install files open the DefaultSetup file within either the x64 or x86 directory, cut and paste, and send you the text for the SQL Server edition you require. It should look something like:

;SQLSERVER2008 Configuration File
[SQLSERVER2008]
PID="AAAAA-BBBBB-CCCCC-DDDDD-EEEEE"


No need to copy/download the whole ISO.