Imagine a situation where you took the card details, but they had to login again after the free trial to keep on using it - re-approve the payment as it were.

You would know then that those customers really wanted your service.

After that it really is just a question of degrees about how far you are prepared to go to make money off people's procrastination.

The fact you ask suggests this troubles you - so be as upfront and honest and clean and clear as you can. And maybe cut back on the food bills...