Definitely worth paying $7 shipping. The T-Mobile Cel-Fi Signal Booster improves signal in your home.
They’re also sold on Amazon: http://amzn.to/2fdf9ex
You must get at least 1 bar of 3G or 4G signal in order for it to work. The signal from the window is rebroadcasted to a satellite device in your home where there is weak or no signal and your phone will work exactly as if it was directly connected to a T-Mobile tower. This booster only works for 3G and 4G devices.