I purchased a License key a while ago in an Amazon UK sale (26 November 2018). I have not yet redeemed the key and now have finally remembered to downgrade my account and then entered the key, only to be told "That licence key has been invalidated. If you’re having trouble, please contact support."
I have been a Plus user since 2016 but for the past couple of days have been on the basic plan (since I wanted to cancel the subscription and let it lapse so I could enter my license key. I therefore don't have access to email support.