Oh no! Sorry to hear you're leaving the family. Sorry to hear even more that you're having billing issues. That's definitely not the T-Mobile way. Here on this support for, it's comprised mostly of fellow T-Mobile customers and a few moderators. None of whom will have access to your account, so we wouldn't be able to check and see what's going on with the bill.
I'd recommend trying to get in contact with the T-Force support team, either via Twitter or Facebook, as they have a secure channel to verify your account, and would be able to look in detail as to what's going on with that final bill.
You can find T-Force on Facebook here: T-Mobile - Home | Facebook and via Twitter here: T-Mobile (@TMobile) | Twitter . The other option of course is to contact customer care by calling in. If you still have a T-Mobile branded device you can call from it using 611, or you can call 1-877-453-1304 from any phone to get in touch with them.