Compare Free Shipping Cell Phone Accessory Deals
A best bet is the Zagg Lumia 710 InvisibleSHIELD Dry Screen Protector (pictured) for $6.95 before the above discount, $6.25 after. With $2 for shipping, that's the lowest total price we could find by $4. It features front and back protectors.
Update: newegg has sold out; however, Groupon still offers it for $44.99 with free shipping.
Need something more colorful? AccessoriesGuyStore via Amazon offers it in Pink for $2.75 plus $2.50 for shipping. That's a price low for this color by about 25 cents.