Stamps.com Total scam Internet

You obviously misunderstood they didn't charge her for two months. They charged her 15.99 twice every month for more than a year. Why would anyone need two subscriptions to the service (or rather non service). Your condescending tone implies that you don't actually know "how the world works" when it comes to scammers such as these. Calling their number means hours on hold to be connected to someone who speaks a barely understandable dialect of English and to be mysteriously disconnected when the purpose of the call is made apparent. Then you get to try it all over again.

Their service is a monthy fee based service.

 If your daughter didn't use the service, that doesn't excuse you from paying for it. You were signed up for 2 months and you paid for 2 months. That's not a scam; that's how the world works.

 I don't know what "service" you were expecting that they didn't provide. It's not something of wizardry--it's a way to print first class mail postage instantly in denominations that would otherwise require a trip to the post office or a week or two long wait for stamps by mail. If you or your daughter didn't use them to print postage, that is, again, not their problem.
As for it being "hard to cancel", it really isn't harder than closing a bank account. You call the number on the website (just like it says to do) and tell them you want to cancel your account. They'll offer you deals to talk you out of it, you say "No, I want to cancel". They cancel it. Again, not a scam, just how the world works.