Quote Originally Posted by Alhanelem View Post
you can just buy a regular copy of SoA and install it- this should work without any issues.

The install path will (by default) be: C:\Program Files (x86)\Steam\steamapps\common\ffxi\

Since steam doesn't provide the game updates for FFXI unlike most games, it should go off without a hitch.
I can confirm that for users of the steam version, if you bought the expansion from amazon digital, the installer installs straight to your *ffxi* folder and the game updates after installation.