Companies that create apps like the one nFusz is c
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Keep in mind there is a BIG difference selling B2B vs B2C. If you are giving away something for free that is very useful, everyone will be all over it. You won't have to market it.
Also consider this isn't just a cool feature like Boomerang. This is an app that will help you make more money selling. If you are selling $100+ worth of stuff a month, this is a no brainer if it helps you.