Charge incorrect amount

I purchased a House of York clothes horse at Pick 'n Pay Hypermarket in Northgate Mall on 15 November. The display price was R389 but the item rang up at the till as R489. I pointed out the discrepancy to the cashier and price was changed to R389. However, Pick 'n Pay did not honour its "double the difference" policy. The supervisor claimed that the policy did not apply as the display price was not barcoded, just signage. This seems to be a feeble excuse for reneging on store policy. I want Pick 'n Pay to refund me R200, which is the "double the difference" amount due for initially being charged an incorrect amount.