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 scroll over "Offers" (for illustration purposes only):
For one-off orders, you can see it upon Checkout when you scroll over Offers (for illustration purposes only):
Do take note, you can only use 1 voucher per transaction.