Answered question
I have cancelled my plan several months ago, but I still can't remove my credit card, because "You cannot delete this payment method because it is associated to an active plan."
And now I've received an email with bonus.
WTF???
I would not feel comfortable with my credit card associated to the account after wanting to cancel.
Contact Fizz support directly and see if they can do something at their end.
Other answers
If you cancel your plan must be inactive