For anyone with a beta account, if you would like to keep using it after the end of the trial period, there is now a button to do this from the Joplin Cloud home page:
If you click on it you will be sent to the Plans page via a special link. Then once you click on "Buy now" you will be sent to the Stripe page where you can start the subscription.
As mentioned in the message, the process takes into account your remaining beta trial days. So for example, if your beta account expires in 60 days, the subscription will have a free 60 days trial period. This is so you don't lose any of the beta trial days no matter when you start the subscription.
If you have any question about it, please let me know.