Just for you promotion

Bought an air time for R55,00 converted R29,00 to data. I then received an sms saying that I can get 100Mb for R10,00 to use for one day. I then bought it as well to my surprise when I cheecked my remaining balance after downloading what I needed to download I realised that my 30 days 100 Mb was gone and left with 71 Mb which will be expiring at midnite. I called vodacom and they told me that that is just how it is. I asked them that I need to lodge a formal complain and they told me that I can only do it over the Internet. I asked for reference number and I was told that there are no reference numbers as their calls are recorded. I should use my cellphone as a reference number which is ***. How is it that a promotional data will only be used after the airtime which is not a promotional one is finished?