Pathetic customer service

I bought a bag and a perfume on 03/10/2019 at the Truworths store in Pietermaritzburg store in town. I used a R150 voucher that they gave me if I bought for 750 and above. I used the bag on Saturday and as I was clipping the long strap on it, the hook broke. I returned the bag today for the same one and they told me the voucher I used is expired therefore I will pay full price. I mean, it's their factory fault I never intended to return the bag but had to because the strap was faulty. Their customer service is pathetic, this is not the first time but I've overlooked all the other instances. I'm fed up i actually wana pay my account up and close it.