You can redeem vouchers with your Perk Points. You will see the vouchers available to you based on how many Perk Points you have.
For subscription orders, you will see this on your Dashboard when you click "Offers" (example voucher shown is for illustration purposes only):
For one-off orders, you can see it upon Cart when you click "Offers" (example voucher shown is for illustration purposes only):
To redeem a voucher, click on the "APPLY" button next to the voucher that you want to redeem. Do take note, you can only use 1 voucher per transaction.