You'd spend far longer in the post office and the courts than you would on the phone.
All the company has to do in that court room is show that you mailed the form to someone whose job has nothing to do with account services, or who doesn't even have access or authorization to access people's private account information, or that they tried and failed to reach you in order to verify your identify since they sure as hell won't cancel the account of someone and shut off their service all because an anonymous letter showed up somewhere nobody was expecting to get it. What a great way to DoS an enemy or competitor if that worked. Just mail off some letters to a few store's webhosts right before Christmas or black friday.
All the company has to do in that court room is show that you mailed the form to someone whose job has nothing to do with account services, or who doesn't even have access or authorization to access people's private account information, or that they tried and failed to reach you in order to verify your identify since they sure as hell won't cancel the account of someone and shut off their service all because an anonymous letter showed up somewhere nobody was expecting to get it. What a great way to DoS an enemy or competitor if that worked. Just mail off some letters to a few store's webhosts right before Christmas or black friday.