I'm looking for suggested apps to install which would allow us to add a single free item to a cart when a group of certain items are added. We would create that free item as a product but hide it to customers on the site so they can't find it and attempt to add it on their own. We need to create the free item as a product so that it has an associated item number for our warehouse to pull. Anyone have any suggestions on apps that would allow us to do that. We've tried some of the bundle or upsell apps and they don't seem to work.