This seems like a good space for an online service to get involved. Certified mail is often a good way to get through to companies with practices like this. They handle it differently because the fact that you are sending it via certified mail usually means that you are doing so to provide legal documentation.

It should be easy enough to setup a business that sends cancellation letters for you with the proof you need if there are any issues. $10 to cancel your internet. $20 to cancel your NYT subscription!