TFG Overcharging

So I opened an account at sportscene and I was told to pay a 15% deposit the first time I purchase and I did which was R285. The next day I purchased I had to pay another deposit of 205 and I asked the cashier why do I have to keep paying a deposit every time I purchase she told me it's a law, I've tried looking in the terms and conditions for what I need to keep paying a deposit and couldn't find anything. Moving forward the tgf company should make customers understand what they putting them self into instead of keep paying a 15% deposit and don't know what the hell is going on.