Excuse my ignorance, but what are packing cubes? Keep in mind I live in Northern Manitoba Canada, far away from any kind of shopping!
It isn't ignorance they are like packing boxes to separate your things. They are either mesh or nylon and they usually have a zipper. I personally don't llike them.
here are a couple of links so you can get a better idea of what they are.
http://www.luggageonline.com/product.cfm?product_ID=14726
http://www.luggageonline.com/product.cfm?product_ID=14725
http://www.worldofbags.com/rick-steves-packing-cube-set-rsk-52?sc=9&category=228313