All coupon codes can be applied on the Payment Details step (Step 2) during the checkout process.
Look for the "Got a coupon code?" link here:
Click it to open a text field where you can add your coupon code and validate it:
Once you validate the coupon, you will see a green "Valid" message. You will also see the new discounted price (reflected below the orange button).
If you have questions about a coupon (or need one 😉), send us an email at [email protected].