There are a world of Android tablets out there, all with a slightly different Android on them, as skinned by the tablet maker usually, which can break things that Android apps rely on.
What is the make and model of this tablet, and what version of Android is it running?
Do you have trouble with any other apps, maybe Netflix or Amazon Prime in the related field of video apps, or any likely unrelated apps?
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.
