The Volume Discount option doesn't apply to multiple sceneries, but only to multiple licenses of the same scenery.
It's thought mainly for home cockpit builders that need to run a product over a network and, by purchasing multiple licenses is a way to get more activations in order to be able to install on multiple machines.
The option that grants a discount when different products are purchased, is the Cross Sell, which is explained here:
http://www.fsdreamteam.com/forum/index.php?topic=2159.0This option doesn't work across different publishers so, Cloud9 products are not included.