I've done this repeatedly (use certified mail, or recorded delivery, the term varies by country) and it's dead easy. I have cancelled service with no early termination fees at both Sprint and Verizon, and kept my subsidized phone while continuing on month-to-month service when they raised fees - a 'materially averse' change - always read the fine print. Same thing with AT&T dial-up back in the day.
This service looks interesting but really there is no easier way than sending a proper business letter and keeping proof of delivery. Only once did a company dispute receiving my letter and I emailed a scanned copy of the proof of delivery and that was it.
It sometimes helps to remind the company that falsely reporting a bad debt onto your credit record carries huge fines and you will definitely be checking to make sure they don't 'accidentally' forget or lose your paperwork, and that you have complete documentation and are prepared to defend. Be polite but do be firm, too.
I can say that if I were running a $5 'let us cancel for you' deal I'd damn sure be doing it all by letter though. No way I'd pay to have a bunch of employees sitting on hold with Comcast when I could crank out form letters and mail them in big batches. Possibly easy money if you have enough clients to scale it.
It would be great to have a repository for such templates on GitHub with variants for different companies. Maybe as Jinja2 files so filling in the sender's name, address, etc. would be easy.
Note that this works best for small-time stuff like broadband or mobile phone plans, if you have a lot riding on it you are better off talking to a lawyer and paying for an hour or two of their time.
Sample text,remember to keep it very short and to the point:
Notice Of Cancellation Of Service (or alternate subject line, use bold print this line only)
Dear Sir or Madam, (or preferably a name if known)
I am writing to cancel service under account number <?12345?> in my name at the above address effective <insert date>.
(optional paragraph) I will return your equipment to your office on the same date and obtain proof of delivery.
(optional paragraph) I reject your rate increase of <insert specific detail here>. You forfeit the Early Termination Fee due to the following materially averse change to our previous agreement: <insert reason here> (It CANNOT be a sales tax increase or anything outside their control but can definitely be an increase in any silly and vague 'regulatory' fees, etc. A classic example is an increase in SMS fees from 5¢ to 10¢, regardless of whether you actually went over your allowance and paid an overage yet. They may offer to waive this but you are not obliged to accept - they chose to alter the deal and they broke their contract, not you)>
<name & signature>