Woot has Hydration Packs on sale for prices listed below. Shipping is free for Amazon Prime Members (must login with your Amazon account) or is otherwise $6 per order.
Thanks to Deal Hunter StrongWeather642 for finding this deal.
Note: If you are checking out with Amazon Prime and you are seeing a shipping cost during checkout, look for and tap/click the "Use this address" button. The order page will finish loading and shipping will update.