With orders of $25 or more, Amazon offers the Munchkin White Hot Safety Bath Ducky for $1.99 with free shipping. (Target charges the same, also with orders of $25 or more.) That's the lowest price we could find by $2. It features a safety disc that reveals the word "hot" when bath water is too hot.

Note: This is part of the Amazon "add-on" program and can only be purchased with orders of $25 or more.