Don't use them, they're crap. End of story.
If you're in a rush to get something, or even want the item at all, go somewhere good, quick, reliable - you know, like Play or Amazon. Hell, it might even be quicker to drive across England to obtain the item.
I've had 2 vouchers from cd-wow for the past few months. I've ordered CDs/DVDs, had the orders delayed, delayed, cancelled, refunded vouchers, ordered new things, delayed, cancelled, refunded.
I've just ordered 300 out of desperation of using another refunded voucher, but not having anything listed (out of their limited crap stock, which probably isn't even in stock) worth buying. So I've figured I might as well order something unworthy of buying with the voucher.
It's not like I'm expecting to receive the film anyway.
I'm also almost up to another month of waiting for the goods from the order of my last voucher. No doubt they'll cancel and refund that soon and I'll have another pointless £20 voucher that I still have nothing to buy from their pointless site.
Roll on once they're used so I never have to order anything from their crap site again.
PS: It also takes about 5 days or more for their customer services staff to reply to any of my "queries". I'm not making this up.