Were you month-month? If so, could be an amount.that.VZN has determined that you owe...for your device. If you disagree, you can dispute the charge and see if the the charge is dropped.
Is this what you are normally charged each month? Did you cancel service before completing your contract?
When is your bill closing date? When did you initiate your cancellation. If you cancelled service after your bill closing date but before you normally receive your bill, you would still receive a bill(and have to pay) for a full month of service.
If you cancelled service before completing your contract, you would receive a bill for the ETF on that line.