How does adding on items/removing items work?

With your first box, you can decide what you want to have extra in the box (t-shirt or replace the mini-figure with the prize figure). These choices are finalized when you are billed. You cannot change them for the box that you have already been billed for. To change them for upcoming boxes, you need to update your preferences before the 1st of the month. Once you are billed, you cannot change what you are getting in the box.