With orders of $25 or more, Amazon offers Prime members the Outward Hound Kyjen Ripstop Large Dog Life Jacket, model no. 22021, for $6.21 with free shipping. (Non-members can spend $49 or more to qualify for free shipping.) That's the lowest total price we could find by $5. It features a top grab handle and front float to keep the your dog's head above water.

Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.