The price you see on checkout is not the price you pay

I ordered 2 pairs of trainers through the website on 13 August 2018. At checkout the system gave a discount and I paid the amount due. I received confirmation emails of the order and of the payment. When I checked my order status this afternoon it was cancelled by the company with no reason for the cancellation. When I called the call centre, they indicated that I must reorder as there was a problem on the website, I asked the call centre agent will the price be the same as per previous order, he said yes the price will be the same at checkout. When I reordered the trainers the price was different. 2nd call to the call centre, they said the discount was not correct and I must pay the full price. The floor manager called me back the same story the website gave the wrong discount and I must pay the full price. As far as my knowledge the price you see, is the price you pay. Now they have lots of stories on why they can’t honour the original price with discount, the manager told me they can offer me a R200 voucher for the inconvenience. I want the trainers for the price that was given to me on checkout initially. It is not my fault that they gave the incorrect discount.