Any NOW TV Membership account matters need to be taken up with NOW live chat due to this being mainly a customer to customer Community Forum.
I am assuming you cancelled fully by following the four steps on screen ?
Usually when a TV Membership is cancelled you get an email from NOW, did you receive any email ?
When cancelling with NOW it's not a good idea to cancel within the last 24 hours when a payment is due or a monthly subscription is going to start or renew when the NOW automatic software payment collection system is running in the background (not saying this happened in your case).
If you have taken the same free trial in the past on the same account, then you will be charged, but it shouldn't be showing available on your online account.
Anyway being a customer just like you, your best bet for a refund is NOW live chat.
You can reach live chat by clicking on the green chat online button from this link page below.