The thing is, you're not actually powerless here - most services do mess this up at some point, and there are multiple ways to fix it. I had something similar happen with a service I'd canceled back in 2024, and the charge showed up a couple weeks after my cancellation window closed. I had the email confirmation too, so I went to my bank first rather than playing phone tag with customer service. They disputed it on my behalf within a few days, and the company didn't even contest it because the cancellation proof was solid.
Your cancellation email is genuinely your strongest asset. When you contact customer service, lead with that and ask for a refund directly - most of the time they'll process it immediately because they know they're on thin legal ground. If they push back or ignore you, then escalate to your bank or credit card company and file a dispute. You don't need to accept their "let's investigate" runaround. Frame it as an unauthorized charge after you terminated the service, attach your cancellation confirmation, and let your financial institution handle it from there.
The other advantage you have is that this charge pattern is documented now. If they somehow try it again, you'll have an even clearer case. Most companies' systems genuinely glitch on cancellations, so this often gets resolved in one conversation. But if you're dealing with someone deliberately difficult, the bank dispute route usually wraps it up in 5-10 business days. Don't waste time going in circles with their support chat if the first attempt doesn't work.