MTN and Mondo Online Contract

I'm not happy with the service that I received from Mondo and MTN , they sold me a contract of a uncapped Data Router which we agreed on paying R499, after delivery, they sent me invoice that was reflecting deferent amount of R700, and I call them to query about that order that we did not agreed on that amount and I want to cancel this order. They don't want to help me to cancel that order, they want the amount which we did not agreed on, they busy telling me that I should take this order to any MTN store and after that I will get a reference no to cancel and I did try to go to deferent MTN Stores I even wasted a lot of money on Petrol traveling looking for MTN Stores and every MTN Store that I get they told me that they just franchising they can't help me, I was so disappointed. After so much money I wasted busy trying to get that MTN Store. After that I called them then they promised to organize the Courier to collect it. But since last week still nothing. I think they try to delay the process of collecting the Production so that I will be forced to pay that amount. And I haven't even used their product. And the next thing it will cost me to look bad on my Cradit Score. While everything was fine, before this mess they put me through.