Double Billing

My contract with Cell C expired in Februaury 2015. Suddenly the prices skyrocketed and I went in to the Cell C store in Greenstone to sign for another 2-years at the cheaper rate. What they neglected to say was that because the contract had expired and even though I would still be on the same number etc. they were effectively going to charge me the higher rate still for the month of April and that I would also be charged for the period in April (in this instance from the 11th of April) for the new reduced rate. Effectively I have had to pay double contracts for the month of April. Call the Call Centre and I am told I need to go back to the Store and take it up with them. Really Cell C...this is what you call customer service??? It is great that you offer the best package for data, but can you blame your customers for leaving you?