How do I cancel SyncUp Tracker service

I received a SyncUp Tracker as part of a promotion.  A few days later, I went on a trip overseas.  Three days in to the trip, the tracker died.  It powers on when plugged in to the adapter, but dies when the power source is removed.  The battery will not accept a charge.  Returned home two weeks later.  I went to a T-Mobile store to exchange the tracker.  They would not do it as it was beyond 14 days of purchase.  I was told it was under warranty, but I would have to pay $5 shipping for a replacement.  $5 to replace a defective tracker?  While under warranty?  I wish to discontinue that service.  How is that done?  The store employee said it couldn't be done at the store and gave me a number.  The number does not work.