With orders of $25 or more, Amazon offers the Exo Terra Aquarium Moss Ball for $3.36 with free shipping. That's $2 under what most third-party sellers are charging via eBay. It works as an aquarium deodorizer, trapping phosphate, nitrite, and nitrate. It's effective for up to two months.

Note: This item is offered as part of Amazon's "add-on program" and can only be purchased with an order of $25 or more.