It's very rare any font is "free" even more so some of the "free" ones aren't open source meaning you aren't free to use it as you please.
Some are free to download and print with via 1 computer; but the moment you take the font file and embed it in software, hardware, put it on a website with @font face or any CSS, or install on a server...you crossed over to commercial use which 99% of the time cost anywhere from a few hundred to tens of thousands of $'s per year. (depending on how widely you are using the font)
Just hit up Monotype they own every font you interact with through any Windows, mac, android or iOS device. (yes MS and Apple DO NOT own the fonts) They pay Monotype to distribute the fonts just like you are looking to do with you product.
Starbucks, NBC, Viacom and a few others have been sued for 6 to 7 figures and lost every time when it comes to not using fonts within license. It'd be like downloading a free copy of angry birds and then changing the name to angry dogs and selling it as your game. You'd have to license their framework because you do not own it.