Where are you trying to buy a pass from?
If you are signed in on your device, simply start watching something, and that action alone will buy the pass.
Or is the problem something else?
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.