Unable to Cancel contract

Is it so hard to cancel a contract? I upgraded one of my month 2 month contracts from the 10 meg to the 20meg . I handed the router into the store and they never sent it away claiming they needed the original paper that came with it . Why don't you guys keep a copy of the original invoice if we loose the invoice As a result I am paying R289 for a router I don't have or even own now.