Ensure you have downloaded the latest version of the Boost Juice App on your mobile device.
Open the Boost Juice App and click on the Vibe tab. There is a multiple choice challenge question, which if answered correctly will give you $2 off. Expires tomorrow so be quick.
It looks like Boost has removed the ability to earn Vibe points with these discounted purchases which is really disappointing. It was pretty much the only time I used to buy their products (when they were on sale, to earn Vibe points). They have also removed the order history from the app.
You also used to be able to email support with Boost gift cards and they could add this credit to your account, not sure if you can still do this. Was handy if you had access to discounted gift cards.