Poor Service

On Sunday 07/02/2016 i used a R100 voucher which i received via post from Uzzi. The voucher expiry date was 07/02/2016 therefore i used the voucher on that day. Tuesday 09/02/2016 i went to return the item purchased as size was not right , plus store did not have size required so i was taking something else instead with the same value. Cashier Selene processed the transaction and i saw that my voucher was not added to my bill. I questioned it that's when cashier called head office. Lady at Head office as they say it was stated my voucher expired and they cant do anything but i am exchanging item for another with same value so what is the issue. i am an existing account member for many years and shop alot in stores using vouchers and first time i ever came across this situation. i must add that i am very disappointed and upset with this voucher story.