If you placed an order using a gift card, any remaining value on your gift card will be saved as credit towards a future purchase with your Zalando account.
Here you can also redeem your gift card and add it to your balance:
If you return items you have purchased with a gift card, the amount of the returned items will be credited to your Zalando account.
Do you have a promotional voucher or discount code?
Promotional vouchers can only be redeemed at checkout when ordering and cannot be saved as credit in your account.