EULA idea for me is best. You can't read the EULA til you open and attempt to install the game, so they can't possibly complain about the fact that the packaging has been removed. Try that. Most places these days refuse returns for PC games on the grounds of having an unsuitable computer, as you are told to check the specs before hand. The Mac areguement is even worse, if you had a Mac why on earth would you open the packaging before realising it wouldn't work?