This does not sound right to me. Is your contract a Mychoice 75, or a Mychoice Topup?
The Mychoice Topup Contract is a semi-prepaid contract. I got one myself. Your bill should to be the same every month.
The cost on my contract is R75 + Caller ID cost. It works out to about R81 pm.
The only additional cost is when you load a data bundle.
E.g if I load a 100 MB data bundle, the amount would be R181.
If you use more data than 100 MB data, the additional cost of the data comes off your available airtime. If you airtime is depleted, you should not be able to connect to the internet anymore.
Then it becomes necessary to "topup" your airtime, the same way that you do it on prepaid.
On the Mychoice 75 contract you would be billed for the extra data.
The bottom line (I think) is that they cannot bill you for that amount on the Mychoice Topup, but it is possible on the Mychoice 75.