So I just signed up for eHarmony and there are two things that catch my eye about upgrading (you need to upgrade so you can see matches, etc.)
The cost. It’s 2-3 times as expensive as any other dating website I’ve seen. Very expensive.
To cancel, you have to send mail, or a telegram, to notify them of cancellation. Mail is understandable, if an onerous requirement, but a telegram? People haven’t used those things in many, many years. Does this sound suspicious to anyone?
Mail would only be understandable if they required you to join by mail. Probably if you want to cancel you should also remove your credit card information.
If you google “how to cancel eharmony” there appear to be other methods, they are just buried. Many people think they wrote mail and telegraph in an outright manner to discourage quitting.
I’m not sure you can even still send a telegram in the United States. I heard years ago Western Union shut down their correspondence telegram service; since then they exist more or less just to transfer money between bank accounts or people.
Western Union shut down their telegram services in 2006.
But fear not! iTelegram (not an Apple product) is alive and well and ready to deliver your telegrams.
I did eHarmony several years ago, and when I cancelled it was relatively easy. Just logged into my account, and there was a choice for “Cancel Membership” or something like that. I don’t remember it being particularly hard to find, but maybe their interface has changed since then.
Of course, I sent a follow-up telegram, to be sure, and had my footman hand-deliver a cancellation notice, to be doubly sure. 
It’s been years since I was on it (I met my husband there!) but it’s possible that you can suspend your account, or suspend matching, or whatever it takes to be charged for the service, without actually closing your account. Then the account lies dormant, ready to be taken up again when things go south.
So the telegram/letter cancellation thing might be about actually deleting the account, and not about ceasing payments.
I belonged to a gym in the 90s whose policy was to cancel you had to provide a doctor’s note stating you could no longer use their service plus a certified letter to their headquarters in Washington DC. Eventually they let up but when I first joined that was how it worked. It’s all about people saying “Screw it, I’ll just pay the $xx per month”.