Refusing to honour a discount voucher

I am based in England. I used NetFlorist to send a Mother's day gift to my Mum in Cape Town. NetFlorist offered at 15% discount voucher to me, which I used to buy a Father's day gift for my Dad, also in Cape Town. When I paid the latter bill, the discount voucher was clearly shown at total paid however, the receipt showed that the discount didn't go through. When I asked NetFlorist to refund the R35.99 difference, they refused saying that I could always use the voucher another time, which I pointed out was useless to me in England! They are still refusing to refund my R35 and their short, unhelpful attitude is disgusting!!