Where are you wanting to display just a single product? In a module or a joomla content item? If a content item is what you are after then the mambot product snapshot will help with this.
If not let me know, what and where you want just one product showing and I will have a look at the modules and see what I can find. I have never tried to have just one item showing all of the time other than in a content item, so I have no idea if it could be accomplished with the current modules. As normally a random selection is good.