Errors at checkout

I ordered a skirt worth R549 yesterday morning. Last night I got an email that the skirt is out of stock with a voucher number. Disappointed that I would now have to spend money on something I might not want instead of just being refunded on my credit card, I had a look and found some stuff I'd like to the value of R599 today. I added the voucher code, made sure it reflected (amount owed showed R50) and checked out. Lo and behold, another R599 was deducted from my credit card. This is not the first time that the voucher codes have given errors at checkout. I've been loyal Zando supporter for very long but I'm getting really sick of struggling with things that should have been teething problems. So what now? Forced to shop AGAIN?