Woof! Woof! via Amazon offers Prime members the HotHands Hand Warmers 40-Pack for $14.99 with free shipping. That's 38 cents per hand warmer and the lowest price we could find by $10 for a similar pack. They give up to 10 hours of heat, last six to ten months, are air-activated, and odorless.

Note: It's temporarily out of stock but still available to purchase at this price. (It's expected to be back in stock on December 7th.)