The Gift Cards feature allows you to create vouchers that people can use to pay for goods. The gift cards can be set up for single-use or multiple uses, and the remaining balance will be remembered after each use.
Follow the instructions on the 'Gift Card Upload' screen. You can then view your gift cards on the 'offers and extras' screen.
If you are using tax on your site, and the customer pays using a gift card, it is possible the subtotal will be less than zero. This ensures that taxes can be correctly accounted for by both supplier and customer. The subtotal plus the tax will make a total of zero.