This saying is an Americanism, and means the item is free. Adding 99 to it is supposed to be a joke because it seems like everything you buy is $x.99, so free.99 is making fun of normal retail pricing.

From the Urban Dictionary (http://www.urbandictionary.com/define.php?term=free%2099): Free 99 - A slang term for something that's free. Ejemplo: Ill give you this fist for the low low price of Free 99. also: When you get something for free. It seems like everything ends with $xxx.99, so then that's the term, free.99 "I got this for free 99."