With purchases of $25 or more, Amazon offers Prime members the HotHands Hand Warmer 10-Pack for $5.60 with free shipping. (Non-members can spend $49 to bag free shipping.) That's the lowest price we could find by $5. Each warmer purports to last up to 18 hours.

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