Did you go through the 4 steps on screen when you cancelled the trial.
Did you see a cancelled message in red text below the Boost Membership when on your online account ?
Normally when you cancel a NOW Membership, you usually get an email cancellation notification from NOW (i haven't used a free trial in about 7 years, but i am assuming it's the same).
Did you receive the NOW cancellation email ?
Your best option is to jump over to NOW live chat and ask them to cancel the Boost Membership and give you a refund.
Here's the link to reach live chat (click on the green chat online button) to start a web chat with them.
Assuming that you did Cancel it successfully, and didn’t run into any of the issues that @schnapps has laid out, cancelling on the 6th day can still be too late, as even though you get the whole 7 days free, Now often have to start the billing cycle on the 6th day in order to meet the banks’ deadlines.
It’s best to cancel a free trial straight away, therefore.
Which perhaps sounds like odd advice, but it will still run for the full 7 days; what you are really cancelling is just the renewal.
And if you decide you do want to continue, you can just unCancel it, and let it run seamlessly into the paid service.