Once you have selected your one-off box and/or subscription, you will continue to the checkout page. If you have a promo code, this can be added underneath the line item as seen here and the discounted amount will automatically be applied and displayed.
If you need to remove an item from your cart, you can do so by clicking on the red “X” seen here.
If you are outside of the US, you’ll want to apply the shipping address first in order to see what the shipping charge would be for your order. This can all be done before you click on the “Checkout” button. This will help to avoid any accidental purchases.