I tried changing the email address under accounts/email and password to match the other subscriptions, but it is not letting me. It says user name already in use. Strangely, they both have the same user name (gmail account) but the new subscription (cloud backup) has a different email address (@mac.com). Even though I am not changing the user name, it won't let me change the email.
Your purchase was made on a me.com domain, I found your other email account that has your subscriptions tied to it which is your gmail account. I transferred the cloud backup to your gmail account. Subscription purchases need to be made using your NC Software account where other subscriptions live, if not then contact us and we can transfer them. It should work now assuming Logbook Pro is signed in with your gmail account.