The return policy on the slip

On the 05-09-2015 I went to buy iron sheet 4.2 mx0.4 mm 10 of them. Then my builder came on the 06-09-2015 to find out we bought the wrong size of sheet. We went to the supplier with him and they say they don't have the sheet he wants. We should bring them back for refund. Of which we did only when I got there the lady told me am not getting the whole amount back according to their policy. As on the slip says refunds and exchange will be charged at 8.5%. I paid R 2 200 on the 05-09 & I was refunded R 1 921.50 on the 06-09. It's really shocking how this company operate. Customer have to loose money when she refund something. It make me wonder is this allowed in south Africa?