Bill Shock on a Top Up contract

I took 2 Top Up contracts with Vodacom knowing well that if you don't have airtime then your services are stopped. How do you allow USSD purchasing of data on a Top Up contract? Reason we go for Top Up contract is to avoid bill shocks. To my surprise that's what I got today when I received my invoice and statement from Vodacom. I'm really disappointed in you Vodacom. I have always been your fan but you did me dirty here. We take Top Up contracts for our families and kids to keep the bill low knowing that they will have to recharge once the data and airtime is depleted. Now I have to spend another R600 on top of what I pay when we are even making enough as it is. I can't wait to get the contracts run down.