Unfair billing due to system error

When I run out of data bundles on my postpaid contract number I receive an SMS notifying me that my data bundle has been depleted. When this happens I purchase the once-off data bundles via *111#, I then receive a confirmation SMS telling me that my once-off data bundle has been activated and then continue to use the data. This then gets billed into my monthly payment. However in the month of April I ran out of data 4 times and each time I purchased a once off bundle and each time i received a confirmation SMS saying that my once-off bundle is activated. Due to past experience and the confirmation SMS I was not worried until i received an invoice of 2888 due to out of bundle rates. I called customer care and they told me that when you purchase a once-off bundle it can take between 4 and 24 hours before its allocated. I don't understand why I have to pay for out of bundle rates that i tried to avoid because the system told me my once-off bundle is active but continued to bill me for out of bundle rates? All i received was advice on how to manage my data bundles which I already no and no resolution and I will left out of pocket this month due to a system that failed to allocate data.