With orders of $25 or more, Amazon offers Prime members the Moshi Screen Protector for Amazon Fire Phone for $7.25 with free shipping. That's the lowest total price we could find by $3. (Non-members can spend $49 to bag free shipping.) It's scratch-resistant and features MicroGrid touch sensitivity enhancement.
Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.