Click "My Next Box" in your account page to go to the add-to-box menu.
Click any "OPEN" slot to add a product to your next box. (NOTE: More slots will unlock each month until your box is maxed)
Make sure that "30ml" is selected in order to browse available 30ml bottles.
Click "ADD TO BOX" to add the ejuice you want to the open slot in your box.
You're done! Your juice will ship with your next box. You can make your box ship right away by clicking "SEND NOW."