Bad Vouchers

On the 31st I placed a order online for 2 pairs of sneakers for my twins I have a voucher that would expire on the day, for some reason it would not process the R175 voucher and I just carried on making the purchase as it was late, when I called TFG they said they cannot do anything as the sale was already done, why would you guys give me a voucher if it doesn't work, I could've bought the sneakers from foot gear and I would've had it by now already and R500 a pair, if you can't process my voucher just cancel my sale and I'll take my business somewhere else