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.
August 13, 2014
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.
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."
August 21, 2014
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