What's wrong with cleaning a shower with a green cleaning product, or making your own vinegar-and-water solution for the shower? First, in my experience (and in the aforementioned guru aunt's experience), any cleaning product, green or not, won't work as well to clean the shower. That's important. Second, even green cleaning products must be manufactured, packaged, and shipped to you or your store, and that comes at the price of fossil fuels. Third, I haven't found a cleaning product that doesn't require some kind of rinsing, and that costs water. I'd rather spend that extra water to rinse off myself, thankyouverymuch.
I promise, I don't work for the squeegee industry, or the "Big Vinegar" lobby.