Was this an offer from Now, or from some cashback service?
If the latter, you need to talk to them, not to Now.
Not that you are talking to Now here anyway; we are all customers like you on the Community, so all we can offer is commiserations, and guidance on who to approach 😢
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.