changing my contract from closed to open contract

On 04/08/2016 my wife took out a wifi modem contract from Vodacom for R299 for 10Gig on a monthly basis.I started noticing that my bill was in access of R600 and enquire regarding this.I was informed that this was an open line and that if the 10Gig was finished i started paying the out of data rate.I was never aware that this line was open and informed the Customer Care lady.They agreed to switch my contract to Top up but the monthly bill was now R499.I feel dissappointed because I feel the salesperson should have explained this contract in detail and the way I was sent from pillar to post to try and resolve this issue.On the 29/11/2016 I was billed an amount of R350.68 and for the following day (30/11/2016) I was billed an amount of R330.10.I feel that if the salesperson could have explained the contract in detail I could have opted for a different contract.I am really disappointed in your service Vodacom and I think your customer service is a huge let down.