This also works for gift vouchers, so add £100 worth of gift vouchers to your basket and they will add a free £25 voucher when your order is delivered.
Go via Quidco for 10% cashback, meaning a potential £90 spend for £125 worth of vouchers.
All this confirmed via chat with a Boux Avenue staff member online. Please query it if you are unsure as it doesn't show up in your basket. They confirm when they receive your order that it is £100 or more and add the free voucher to your delivery.