Bad service from Truworths

I'll never ever shop at Truworths again. I bought a dress for my wife 2 weeks ago and the size was wrong, I misplaced the slip and went back to Wonderpark Truworths for an exchange and the refused even with the price tag on it. Why is it that I always return/exchange things without a slip with their competitors but they can't meet their customers half-way? I was even willing to pay extra should what we change to differ in price. Even their customer service isn't helpful. What will I do with this expensive dress that doesn't fit my wife?