I get them wanting someway better than email to make sure I can't repedetedly sign up for the trail, but don't fucking auto-subscribe once the trail is over.
I know its not the point but this kind of stuff happens so please consider privacy.com. You can set a per transaction limit, monthly limit, one time use. Etc. Please take a look as it lets you do some amazing things.
Just use up one of those gift cards you can use in any store and use that number for free trial signups. Then they can't charge you when the trial period expires.