Spar refusing to refund me for returning items.

I went to make some groceries which I thought it was on special pricing unfortunately, the special was only starting the following day. I therefore return the groceries and asked to be reimbursed, they deducted 4% of the the total amount of what I had spent claiming that the bank also charges them for every sale that they make (I paid using my cheque card). They refused to refund all my money, is that right/fair? Can someone who knows better to please help!!!