Once off data purchases

I have monthly 10gig(day) plus 10gig(night) data package and last month i bought an extra 5gig once data bundle. Of the once off 5gig data bundle, i have a balance of 2gig remaining which i cannot use as the new monthly 10 gig data has been loaded and this is being utilised. I am being penalised and ****** by vodacom as the once off data bundle expires on the 20th of this month while the 10 gig only expires the following month. So Vodacom wants me to finish my 10gig data before i can access the once off data bundle of 2 gig by 20th of the month (So the client won't have any data bundle from the 21st until the end of the month) or alternative i must just forfeit the 2gig once off data bundle. Thank you Vodacom for treating your clients this way. You would the think that the normal process would be to allow the client to utilise the data that would expire first of which the client has paid for over and above their normal monthly package. I considering going to your competitor as i am paying R299 per month for this monthly data package and there are occasions when i purchase additional once off data, where as your competitor(MTN) is offering 20gig (day )and 20gig(night) for only R249. I dont like MTN as i have been a long standing vodacom client and i trust the vodacom brand, however to meet my needs and in order to get value for your money and not to be penalised in this way, i am prepared to make the change....