If I understand correctly, you get a catch of the day voucher for every dollar you spend on this particular sale but not including p/h. So essentially you get whatever item for free but the money has to be used for another deal for COTD.
Sorry if I misunderstood it.
Oh and it's until midday AEST on Monday 26/05.
Keep in mind, the voucher is only valid for a month after you get it.