NetFlorist is *****ing my money

I bought gifts for my parents. A few days after paying I was told one of the items was out of stock. I was then advised that instead of a refund they will give me a voucher because refunds take long. I received the voucher, used it but I did not use all of it. So my account was in credit. I mailed Netflorist asking them to apply the voucher to my next order and I was given this reply: " Thank you for contacting NetFlorist.

Please be advised that you can only use a voucher one once you have used the voucher we can no longer reinstate it, hence we advised the client to use all the amount of the voucher. "

How does this work??? I paid for everything IN FULL and the woman who advised me to take the voucher DID NOT tell me to use all of it because the voucher is only good for one purches. I paid them R1000 for gifts, received a R500 voucher for the item that was out of stock but only used R400. Meaning theres R100 in CASH surplus on their side. But they refuse to give it back to me because of the "can't reimburse vouchers" logic. I am extremely unhappy