I'm really sorry about the overdraft. I would call your bank and explain exactaly what happened; you signed up for a free trial and didn't know there was an authorization charge and that the charge which led to the overdraft was just that - an authorization. To clear some things up... Stamps.com is NOT a scam and has been a service that I have enjoyed for many years. I've canceled before (I didn't join through fusion cash for those who will start jumping up and down) and simply called them. Free trials like Stamps.com often carry small authorization charges; I just don't understand why everyone is freaking out and suggesting that a completely legitimate service is a scam. Everyone just needs to calm down!
If you go to the gas station and pay at the pump - even if you buy 1 gallon of gas they'll process an temporary authorization for $50.00. If go to a nice resteraunt, there will likely be an authorization charge as well. If you check into a hotel, they'll often automatically process an extra $50 authorization. Hotels, resteraunts and gas stations aren't scams - authorizations are just that - common, frequent, and a normal way of doing business.
That said, the bank problem with an overdraft is really unfortunate and I understand you didn't know this. I really hope your bank will work with you - try to explain exactally what happened and hopefully they will.
Best & Take Care