Buying a smart phone from a third-party, discount online retailer might seem like a shrewd move, but a $50 discount could cost you $400 later if something goes wrong, in addition to any early termination fees charged by the carrier. That means consumers who buy from big-name third-party retailers like Target or Radio Shack could end up facing up to $750 if they prematurely cancel service. It also means consumers might be hit with a big bill from an unexpected place.