Telkom unfairly throwing away data that I purchased.

Towards the end of last month, I purchased additional data for my contract. Now, in the new month, I still have 5GB of that once-off data remaining. The system is currently deducting from my monthly data rather than this once-off data that I purchased. This means that when the once-off data expires during the course of the month, it will be discarded, and I will probably run out of data again at the end of this month.

Surely data that expires first should be deducted first? I contacted the call center and they said this is just the way it works and there is nothing they can do about it. I simply have to watch the data I purchased get discarded while I run down my monthly contract data.