Add-on items are small and inexpensive products Amazon doesn't normally sell on their own, but lets customers add to their orders of $25 or more to save on shipping costs.
They may sound like a great deal, but if you didn't really intend to buy them in the first place, it's a waste of money.
"While the extra $5 or $6 for a product doesn't seem like a big deal, it can be a major money drain if the product isn't something you really need," Williams said.