Add-on items can only be tacked on to other orders.
How the hell would I know that the item I'm trying to buy is an "add-on" item? I searched online, which linked me to that Amazon item. I clicked to add the item to my cart, and attempted to check out. Nowhere does it say "THIS ITEM IS AN ADD-ON ITEM". It tells me in the top right that add-on items need to be part of a purchase of at least $25, but not that the item I'm trying to buy is an add-on item.
*EDIT* looked again at the screenshot, and apparently there is a blue flag thing under the item name saying "Add on item". But again, it's way less clear and concise than it should be, as to what's going on.