What's the point of setting a limit

I have set a limit on my Vodacom account and I am continuously over billed. I don't get notifications whether I am half way through and when my invoice comes it was well over the limit that I have set. I have gone into a Vodacom shop and was told that Vodacom cannot guarantee this service will work. It seems that if Vodacom deems the limit too low they will overbill just to get more money. If Vodacom isn