Online shopping system poorly designed

I'm shopping online at [URL Removed] I select my purchase, enter delivery address, enter gift message, enter my Woolworths Gift Voucher number, I then notice that the address is not displaying correctly so I go back to correct it, but when I return to the payment screen and enter my Woolworths Gift Voucher this time it says it's an invalid voucher and the cash is not on the card. I call customer services four times, I get cut-off twice, transferred to \Tiger Brands"once and the last time they say we'll call you back. But I ask"