Need clarification.

Hi there. This is more of a question than a review since this is the only platform where I am confident that I will get an answer.

I got this reward from vodapay :Your reward

R10 off on VodaPay when you spend R11 or more. I bought data for R12 expecting to get the R10 off and only be charged R2 only but I paid the full price. Can someone please explain to me how what the reward means exactly. Thanking you in advance for your assistance.