If you received a gift voucher code from a friend, there are two ways in which it can be used to offset your order payment.
1) By entering the code in gift voucher field inside the shopping cart and pressing Apply button, as shown in the image below.
2) Or by first redeeming the gift voucher as a store credit and then using the store credit to pay for your order.
The image below shows how to redeem gift voucher as store credit:
You can refer to this support article on how to use store credit to offset your purchases:
Note that Pupsik's voucher codes never expire and can be used partially, split over several orders.