I can't dup this on php4 or php5.

I use
$this->add_twoforone_cat(21);
at

http://192.168.1.103/swguy_demo_1.3.7/
which is php4.

Cat 21 is gift certificates. Add two OF THE SAME and you'll see the discount. I know the marketing text is misleading; I will fix this in the next release.