@Anonymous User
The browser combination is a bit rocky, but the Store app is pretty solid.
Try uninstalling it (a how-to is here) and then installing it again; this sometimes improves things.
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.
