Just dont argue guys... and have a nice day.
I personally think mannatech is complete bogus, due to the lack of scientific evidence that supports its claims.
But still, it is not a possible thing to argue.
Either these guys want your money, and somehow think debating will make you hand over your wallet, or they are very very convinced that these placebo pills are working. Of course, that is kind of the point of placebo. But if this placebo is really what it takes for some people to become happier, and in turn, psychologically feel healthier, why argue? As long as it does not intervene, life is life.
... And stop attempting to argue against flat-earthers or the like. They are just trolling or kinda mentally abnormal.