Clicks online not applying cashback voucher at checkout and online orders cannot be cancelled

I recently placed an order online with Clicks. At checkout I added my cashback vouchers. The system accepted my amount and took me through to payment. Once payment was completed and I received my order confirmation, I noticed that my cashback was NOT deducted! I immediately wrote to Clicks who rep**** that, because the 'cents' of amount of the cashback was inserted, the cashback could not be deducted. Firstly, this was never a problem in the past and secondly, their system added the cashback amount including the 'cents' when I was placing the order (which I proved with a screenshot taken at checkout) - if the amount was not acceptable, why did their system not reject it? It just went straight through to payments. Then I asked for a refund, to which they rep**** - they are unable to cancel the online order.