Forget about this plugin. The author is trying to earn from your commissions without clearly stating so in the plugin documentation. I understand it’s a free plugin, but i’d be more than happy to donate had the author not being so sneaky about the whole thing.
The author’s other zazzle plugin is the same, it doesn’t even have an option to put your own associate id.
If anyone insists on using this plugin, open the file “functions.php” and you’ll see author’s associate id in this file, change that with yours.