I am looking at the buy one get one free add on updated by sandra warne

Just wondering is this on a per product basis or can I do this on different products in the same category?

basically I sell (amongst other things) used games and dvds, so would like to do the occasional 3 for 2 offer, etc.

Any advice would be appreciated, and if this doesn't do that, does anyone know what does?

Thanks in advance!

forgot to mention I am using 1.3.9h